Short Answer:
There has never been much reason to consider England or Britain an empire.
Long Answer:
Part One: A discussion of the nature of empire.
Part two: A brief history of Britain in relation to the concept of Empire.
Part Three: Three very weak reasons to consider Elizabeth II an empress or emperor.
Part One: A discussion of the nature of empire.
I don't think of "colonial empires" as empires at all. They can be called [thalassocracies][1].
thalassocracy (countable and uncountable, plural thalassocracies)
A state whose power derives from its naval or commercial supremacy on the seas.
Maritime supremacy.
And a "colonial empire" can be called something like a "colonicracy", a word I made up, meaning "colonial realm".
And I don't approve of calling colonial realms empires.
You have to remember that for most of western history, people in western civilization didn't believe in empires, they believed in THE EMPIRE. They believed in one, and only one empire, the Roman Empire.
When the Roman Republic grew powerful, and dominated the Mediterranean region, the Romans came to believe that the gods had decreed that they were the rightful rulers of everywhere. They believed that they conquered their empire because they already had the gods given right to rule it, not that they had the right to rule their empire because they had conquered it.
And it seems that the early Christians, despite their occasional problems with the Roman government, accepted that ideology as well.
"[Render unto Caesar][2]" is the beginning of a phrase attributed to Jesus in the synoptic gospels, which reads in full, "Render unto Caesar the things that are Caesar's, and unto God the things that are God's" (Ἀπόδοτε οὖν τὰ Καίσαρος Καίσαρι καὶ τὰ τοῦ Θεοῦ τῷ Θεῷ).[Matthew 22:21]
The synoptic gospels of Matthew, Mark, and Luke were probably written AD 70 to 110, AD 65-75, and AD 80-110, respectively.
The meaning of "Render unto Caesar that which is Caesar's, and unto God that which is God's", seems fairly obvious. That which is God's is not that which is Caesar's, and Christians, especially Christian clergy, can not follow or honor God by lusting after that which is Caesar's. And just as Christians must render unto God, and not unto other gods, that which is God's, they must render unto Caesar, and not unto other secular rulers, the taxes and political obedience that are Caesar's.
In any case, the early Christians soon came to believe that the Christian God had decreed that the Roman Empire was the rightful government of Everywhere and Everyone, and would endure forever.
And for many centuries Christians continued to believe that the Roman Emperor was more or less the rightful ruler of everyone and everywhere. Although there were often disagreements about which ruler claiming to be the Roman Emperor had the better claim.
Part two: A brief history of Britain in relation to the concept of Empire.
I may note that in the reign of Gallienus (r. 253-268) the legions in Gaul revolted and proclaimed Postumus Roman Emperor in 260. this is called the Gallic Empire by modern historians, that included Britain, Gaul, and for a time Spain. Shapur I, King of Kings of Iran and of Non Iran, invaded the Roman Empire, but was defeated by forces led by Odaenathus, leader of the city of Palmyra, who also defeated self proclaimed Emperors in the east. Odanathus was given high honors by Gallienus and in 263 proclaimed himself King of Kings of the East, with his son Herodianus or Hairan I as co King of Kings.
In the Roman Empire's hierarchical system, a vassal king using the title of King of Kings did not indicate that he was a peer of the Emperor or that the ties of vassalage were cut. [Wikipedia:Odaenatus][3]
Odaenatus was not the first or the last vassal of the Roman Republic or Empire to use the title of King of Kings.
Odaenatus and his son Hairan i were assassinated in 267, and Odaenatus's young son Vallabathus became King of Kings with his mother, Queen of Kings Zenobia, as his regent. Zenobia sent a Palyrene army to conquer Eqypt in 270 and another to coquer large parts of Asia Minor in 271. Zenobia had King of Kings Vallabathus proclaimed Augustus and herself Augusta, emperor and emperess, in 271.
Emperor Aurelian defeated and conquered the so called "Palmyrene Empire" in 272. Aurelian then invaded and conquered the "Gallic Empire" in 274.
In 286 Emperor Diocletian chose a general, Maximian, to be his co Emperor, Diocletian ruling the east and Maximian in the west. They used the title of Imperator Caesar Augustus. In 293 Diocletian and Maximian adopted as heirs and junior co emperors Galerius in the east, and Constantinius Chlorus in the west. Galerius and Constantinus used the title of Imperator Caesar, which was thus lower than the full imperial title of Imperator Caesar Augustus.
So if the two word phrase Imperator Caesar was not enough to make someone a senior emperor, but Imperator Caesar Augustus was required, the single word title Imperator should not be enough to make someone an emperor.
In 286 the Roman officer Carausius proclaimed himself emperor, seizing Britain and parts of Northern Gaul. Constantinius Chlorus retook the the lands in northern Gaul in 293 and reconquered Britain in 296, ending the so called "Brittannic Empire".
In 305 Diocletian and Maximian retired, and Constantinus Chlorus and Galerius became senior emperors, with Valerius Severus and Maximinian Daia as their junior co emperors. Constantinius Chlorus died in York in Britain in 306. His son Constantine I was proclaimed Imperator Caesar Augustus by the army in Britain. At first Constantine I was only emperor in Britain, but conquered the rest of the Roman Empire in various civil wars lasting until 324.
The troops in Gaul proclaimed Flavius Magnus Magnetius Emperor in 350, ruling over Britannia, Gaul, and Spain. He was defeated by Constantius II in 353.
In 383, the Roman commander in Britain, Flavius Magnus Maximus, was proclaimed Emperor by the army in Britain. Magnus Maximus gained control over Gaul and Spain and made his son Flavius Victor co emperor. Theodosius I, Emperor in the east, defeated and killed Magnus Maximus and Victor in 388. When Theodosius I died in 395 his son Arcadius became Emperor in the East and his other son Honorius became Emperor in the west.
In 406 the Roman soldiers in Britain revolted against Honorius and made a soldier named Marcus emperor. A few months later they killed Marcus and made a municipal official named Gratian emperor. In 407 the soldiers killed Gratian and made a soldier named Constantine Emperor. The new emperor Constantine III led most of the Roman army in Britain to Gaul, gained control of Britain, Gaul,and Spain, and made his son Constans II co emperor. Honorius's general who became emperor Constantius III, defeated and killed Constantine III and Constans II in 411.
According to Procopius writing over a century later, Britain was never ruled by the Roman Empire again, but by tyrants. And tyrants was the world for Roman usurpers who who claimed the imperial title but were never successful at being recognized as legitimate emperors. Thus there may have been some of sort of (North) Western Roman Emperors reigning in Britain for centuries after 411.
In 475, Julius Nepos, emperor in the west, was deposed by the barbarian soldiers of the Roman Empire in Italy, and fled to Dalmatia. Orestes, the leader of the revolt, made his young son Romulus Augustulus emperor in Italy, while Julius Nepos continued to claim the imperial title in Dalmatia. The barbarian soldiers revolted and killed Orestes in 476, and made their leader Odoacer king. Romulus Augustulus was deposed. The Roman senate sent the imperial insignia to Zeno, emperor in the east, saying that a separate emperor in the west was no longer necessary. Zeno continued to recognize Julus Nepos as western emperor until Nepos was assassinated in 480.
Syagrius ruled a Roman state in part of Northern Gaul, perhaps claiming to be an official of the eastern Roman Emperor or of the hypothetical emperor in Britain, or to be emperor himself, until he was defeated and killed by Clovis, king of the Franks, in 486,487, or 493-4.
In about 496, a man named Burdunellus claimed the imperial title in Spain, but was soon captured and killed. in 506 a man named Peter claimed to be Roman emperor in the Ebro valley in Spain but was soon defeated and killed.
In North Africa, the Berber kingdom of the Aures was in parts of Tunisia and eastern Algeria. A man named Masties allegedly ruled it for 67 years from 426 to 494, or from 499 to 516. Allegedly at first his title was Dux, general or governor, but for the last 40 or 10 years (454-494, or 484-494, or 476-516, or 506-516) Masties allegedly used the title of "Emperor of Romans and Moors". I noted that Romulus Augusutulus was deposed in 476, and Peter was killed in 506. I don't know if Masties's successors used the imperial title, and the Muslim caliphate completed the conquest of North Africa by 708.
With these exceptions, everyone in the west recognized the Roman Emperor in the east as the legitimate Roman Emperor.
The eastern Roman or "Byzantine" empire had a thousand years of fascinating history, until its last remaining parts were conquered by the Ottoman Turks; Constantinople in 1453, the Morea in 1460, Trebizond in 1461, and the Principality of Theodoro in the Crimea in 1475.
As the eastern Roman or "Byzantine" Empire gradually abandoned the use of Latin and became more exclusively Greek speaking, the emperors came to use Greek titles more and more. It became common to call the Emperor the Basileus, which originally meant "king", but came to mean something like "The Only King in all the World". The imperial title became Basileus kai Autokrator ton Rhomaion, which is usually translated as "Emperor and Autocrat of the Romans", but perhaps should be translated as "King and Emperor of the Romans" or even as "Emperor and Emperor of the Romans".
In 797, Emperor Constantine VI was deposed and blinded by his mother Irene, who then ruled the Roman Empire until being deposed in 802 by an official who became Emperor Nikephoros I. Meanwhile, in the west, the powerful Charles the Great, Charlemagne, King of the Franks and the Lombards, claimed that the imperial position was vacant since it was occupied by a female, and had himself crowned Emperor in Rome by the Pope in 800.
And it is possible that an objective observer might decide that Nikepohoros I had much more right to be considered the rightful successor of Constantine VI than Charlemagne did. For example, they might split the right 90 percent to Nikephoros i and 10 percent to Charlemagne. But Charlemagne's successors continued to claim to be the rightful successors of Constantine VI, and of all the eastern Roman or "Byzantine" Emperors back to Arcadius in 395, and of all the classical Roman Emperors back to Augustus in 27 BC.
The imperial title used by Charlemagne Holy Roman Emperors continued until Charles III was deposed in 888, and then was used off and on until Berenger I was assassinated in 924. In 962 Otto I the Great, mighty king of the East Franks or of Germany and of Italy or Lombardy, was crowned Emperor. His realm came to be called the Holy Roman Empire and lasted until 1806.
The usual Latin title of the Carolingian emperors, and the early Holy Roman Emperors was Imperator Augustus, which might be translated as "Emperor Emperor", or as 'Emperor", assuming that the two word phrase meant "Emperor".
Before 1200 it became the rule that someone elected emperor used the title of Rex Romanorum et semper Augustus, "King of the Romans and always Emperor", until he was crowned in Rome by the Pope, when he took the title of Imperator Romanorum et semper Augustus, "Emperor of the Romans and always Emperor".
Meanwhile, back in the island of Great Britain, a Romano-British society existed in post Roman Britain, possibly with a line of (north) western Roman Emperors as the overlords of many kingdoms. In the Middle Ages, a number of rulers were mentioned with the title of "King of the Britons", implying that they were the overlords of the other British kings, and thus possibly the hypothetical Roman Emperors in Britain, successors of Constantine III.
Almost every single man living after about AD 550 mentioned as King of the Britons was also the King of Gwynedd in northwest Wales.
In the 5th century, various Germanic groups settled in Great Britain, and by the time of the mission of St. Augustine in 597 a number of Germanic kingdoms ruled almost all of southern England and much of northern England, with the British kingdoms restricted mostly to Wales, Cornwall, and northwest England. Those Germanic groups were collectively known as Saxons by outsiders but called themselves collectively Angles.
By about 886 the Danes had conquered much of England, and Alfred the Great, king of Wessex, led the rest of the Angles and Saxons, so Alfred took the title of Rex Anglo-Saxorum, "King of the Anglo-Saxons".
Alfred's grandson Aethelstan acquired the Danish territories in northern England and took the title of Rex Anglorum, "King of the Angles" or "King of the English", in 927. And in 1154 King Henry II changed the title to Rex Anglia, "King of England".
However, many Anglo-Saxon kings also sometimes used various more grandiose titles, including the imperial titles of Basileus and Imperator, though not augustus or caesar. Basileus was used numerous times between 930 and 1060, while Imperator was used many times between 930 and 1018.
[Index of the Styles and Titles of Sovereigns of England][4]
Owain Gwynedd was King of Gwynedd from 1137 to 1180. Owain Gwynedd also took the title of Prince of the Welsh. The English word "prince", which has several meanings, comes from the Latin princeps, meaning first.
in the late Roman Republic the Princeps senatus was the first and most senior senator, a position with some powers and much prestige. When Augustus became the first Roman emperor, he acquired a number of Republican titles, offices, and powers, including that of Princeps senatus. So the early Roman emperors often used the title Princeps senatus, and sometimes Princeps civitatus "first citizen", and the early period of the Roman Empire up to about 284 is often called the Principate.
So it is possible that Owain Gwynedd intended his princely title to imply imperial rank. His successors in Gwynedd used the title of Prince, as in "Prince of Aberffraw and Lord of Snowdonia", or "Prince of Wales", up until the final conquest of Gwynedd by the Anglo-Saxon invaders of Britain in 1282-1283, and may have intended to mean "Emperor" by their use of Princeps.
Of course the Kings of England did not acquire any rights to be the successors of the Kings of Britons by conquering Gwynedd. The Kings of England were the successors of over eight hundred years of Anglo-Saxon rulers who were rebels and traitors against the Kings of the Britons and/or invaders of the territory of the Britons. Success in battle could never take away the impediment of having been the enemies of the Kings of the Britons for over 800 years.
And of course after 800 years of hostilities the Kings of England could never acquire any right of succession to the hypothetical (north) western Roman emperors in Britain.
In 1533 the English Parliament passed the Statue in Restraint of Appeals as a step in making king Henry VIII the head of the church in England. An extract from the act says:
Where by divers sundry old authentic histories and chronicles, it is manifestly declared and expressed that this realm of England is an Empire, and so hath been accepted in the world, governed by one Supreme Head and King having the dignity and royal estate of the imperial Crown of the same, unto whom a body politic compact of all sorts and degrees of people divided in terms and by names of Spirituality and Temporalty, be bounden and owe to bear next to God a natural and humble obedience: he being also institute and furnished, by the goodness and sufferance of Almighty God, with plenary, whole, and entire power, pre-eminence, authority, ...
And if any person or persons, at any time after the said Feast of Easter, provoke or sue any manner of appeals, of what nature or condition soever they be of, to the said Bishop of Rome, or to the see of Rome, or do procure or execute any manner of process from the see of Rome, or by authority thereof, to the derogation or let of the due execution of this Act, or contrary to the same, that then every such person or persons so doing, their aiders, counsellors, and abettors, shall incur and run into the dangers, pains, and penalties contained and limited in the Act of Provision and Praemunire made in the sixteenth year of the king's most noble progenitor, King Richard II against such as sue to the Court of Rome against the king's crown and prerogative royal....[5]
[Wikipedia:Statute_in_Restraint_of_Appeals][6]
And the claim that England had always been recognized as an empire, separate and independent from the Roman Empire, was mostly a lie. The theory and ideology of the Holy Roman Empire was that it was the continuation of the eastern Roman or "Byzantine" Empire which was the continuation of the classical Roman Empire, and that it was not "an empire" but THE EMPIRE and the rightful direct or indirect ruler of all the world.
I note that the Wikipedia article on the Act in Restraint of Appeals gives various dates when various parts of it were repealed in United Kingdom or parts of it, and says:
The whole Act, so far as unrepealed, was repealed by section 1 of, and Part II of the Schedule to, the Statute Law (Repeals) Act 1969.
[Wikipedia:Statute_in_Restraint_of_Appeals][6]
That sounds like whatever was left of the act was finally repealed in 1969, and thus that the description of England as an Empire in that act is no longer a part of the law in the United Kingdom. And of course the Kingdom of England united with the Kingdom of Scotland in 1707 to form Great Britain, and so England was no longer an independent Empire after 1707 if it was before.
On 20 October 1721, the Senate of Russia, an eastern Orthodox country, granted the title of Emperor to Peter the great, who accepted it on 2 November 1721. The title was accepted at various dates by various western European Protestant and Catholic countries, the Dutch Republic and the kingdom of Prussia in 1721, the kingdom of Sweden in 1723, the Ottoman state in 1739, Great Britain in 1742, Austria, Hungary,and Bohemia, etc. in 1742, France and Spain in 1745, and the Polish-Lithuanian Commonwealth in 1764.
The Russian Imperial title is usually translated as Emperor and Autocrat of all Russia" Although "Emperor and Emperor of All Russia" might be more correct.
And this opened the floodgates for more and more rulers in European civilization to claim the title of Emperor.
The Russian Empire (1721-1917)
The First French Empire (1804-1814, 1815)
Empire of Austria (1804-1918)
First Empire of Haiti (1804-1806)
First Mexican Empire (1821-1823)
Empire of Brazil (1822-1889)
Second Empire of Haiti (1849-1859)
Second French Empire (1852-1870)
Second Mexican Empire (1864-1867)
German Empire (1871-1918)
Empire of India or British Raj (1876-1948)
Empire of Ethiopia (Italian) (1936-1943)
Central African Empire (1976-1979)
I tend to think of those alleged empires as "inferiums" instead of imperiums.
The proliferation of alleged non Roman empires in western civilization in that era reminds me of the situation in Eastern Europe and western Asia in the period from 1355 to 1371, when the alleged emperors included:
the so called "Latin Emperor", in exile in western Europe.
The heir of the empire at Nicaea, now in control of Constantinople again, and usually called the "Byzantine" emperor by modern historians.
The "Emperor and Autocrat of all the East, the Iberians, and the Transmarine provinces" in Trebizond.
Stefan Uros V "Emperor and Autocrat of the Serbians and the Romans, the Bulgarians and the Albanians">
Simeon Uros, rival "Emperor of the Serbians and the Romans" ruling in Thessaly.
ivan Alexander, "Emperor of the Bulgarians and the Romans", with his capital at Veliko Tarnovo.
ivan Sratsimir, rival "Emperor of the Bulgarians and the Romans", with his capital at Vidin.
The monarchs of the United Kingdom also had the title of Emperor or Empress of India (Kaiser-i-Hind) from 1876 to 1948.
Part Three: Three very weak reasons to consider Elizabeth II an empress or emperor.
One) About 50 or 60 years ago there was reportedly a small cult in Switzerland that was said to worship Queen Elizabeth II as "Empress Saint of the Universe". The reasons for that were not mentioned, but anyone who accepted their validity could thus consider her an empress.
Two) It may be mentioned that the rightful genealogical heirs of the great Salian and Hohenstauffen dynasties of the Holy Roman empire would be the genealogical heirs of the only legitimate child of Emperor Frederick II to have descendants to the present time.
His legitimate daughter Margaret (1241-1270) married Albert the Degenerate (1240-1314) Count Palatine of Saxony, Margrave of Meissen, and Landgrave of Thuringia. Their heir by agnatic (male only) primogeniture is Prince Michael (b. 1946), claimant to the Grand Duchy of Saxe-Weimar-Eisenach. Their heir by male preference primogeniture, in which a daughter can succeed if she has no brothers, is Queen Elizabeth II.
Thus Queen Elizabeth II is a potential claimant to the throne of the Holy Roman Empire.
Three) The titles of the monarchs of the United Kingdom have two forms, an English form and a Latin form.
Since 29 May 1953 the titles of Elizabeth the Second have been.
In English: Elizabeth the Second, by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith[19][2]
In Latin: Elizabeth II, Dei Gratia Britanniarum Regnorumque Suorum Ceterorum Regina, Consortionis Populorum Princeps, Fidei Defensor[20]
[Wikipedia:List_of_titles_and_honours_of_Elizabeth][5]
The two titles are not exact translations of each other. The title of Princeps of the commonwealth of Nations would usually be translated as prince, but as pointed out above, might possibly be translated as emperor, so there is some ground for considering Elizabeth II to be an Emperor, but of the entire Commonwealth of Nations and not of the United Kingdom.
[1]: https://en.wiktionary.org/wiki/thalassocracy#:~:text=thalassocracy%20(countable%20and%20uncountable%2C%20plural,Maritime%20supremacy.
[2]: https://en.wikipedia.org/wiki/Render_unto_Caesar
[3]: https://en.wikipedia.org/wiki/Odaenathus#King_of_Kings_of_the_East
[4]: https://books.google.com/books?id=iRsDAAAAYAAJ&pg=PA49#v=onepage&q&f=false
[5]: https://en.wikipedia.org/wiki/List_of_titles_and_honours_of_Elizabeth_II#The_British_Isles
[6]: https://en.wikipedia.org/wiki/Statute_in_Restraint_of_Appeals#:~:text=The%20Act%2C%20drafted%20by%20Thomas,Wales%2C%20and%20other%20English%20possessions.&text=Also%20King%20Henry%20wanted%20to%20intimidate%20the%20pope.
