Here is a link to a question and answer about Roman Emperors who visited Britain.
Quora: Which Roman emperors set foot in Britain?
My answer has the most complete list.
Of course nobody would ever confuse a Roman Emperor visiting Britain with a with a vassal king of Britain, would they?
Well, there were vassal or client kings, not of Britain, but in Roman Britain, for at least some parts of the more than 350 years of Roman rule in Britain. So I think that the Britons should have been able to tell the difference between a contemporary king in part of Britain, and a contemporary Roman Emperor, even when the Roman Emperor was visiting Britain.
But would the Britons and their later descendants always be able to tell the difference between a client king of the Roman Empire ruling in part of Roman Britain decades or centuries earlier and a Roman Emperor visiting Roman Britain decades or centuries earlier? Or could people mistake one for the other if they lived longer enough earlier.
The political geography of the early Roman Empire was most of the land was ruled by city states annexed by the Romans or formed out of former barbarian tribes and kingdoms. Those city states were ruled by landowning aristocrats, some descended from former kings and nobles, who competed to be elected as magistrates and council members.
Roman governors ruled provinces containing city states, and supervised the city governments as well as defending the province from invasions, conquering neighboring lands, and crushing revolts. And the central government in Roman appointed the governors and paid for the troops stationed in the provinces from various taxes.
And early Christians developed ranks of clergy, including priests and bishops who oversaw the Christian communities in various city states. Eventually the bishop of the metropolitan city of a province gained the power to supervise the other bishops of other city states in the province, and became an archbishop or metropolitan while remaining bishop of his city.
The Roman Empire changed a lot after the Crises of the Third Century. The military and civilian duties of governors were separated. Provinces where split into two or more smaller provinces. The governors of several provinces were put under the supervision of officials called vicars, who were under the supervision of officials called Praetorian Prefects.
Roman Britain was now called The Britains, since it was divided into at least 4 and maybe 5 or six provinces. All of Roman Britain was under the authority of the Vicar of the Britains, who was under the authority of the Praetorian Prefect of the Gauls, who ruled Britain, Gaul, Hispania, and Roman Morocco.
And some of the archbishops began to claim to be more arch than other archbishops. Over centuries the archbishops of Jerusalem, Antioch, and Alexandria claimed to have extra wide authority over other archbishops. The archbishops of Rome, the largest city and the founding city of the Roman Empire also claimed such special authority, and when New Rome or Constantinople became another capital city the archbishops also claimed such wide authority. Those five super archbishops were eventually considered to be five patriarchs and highest clergymen.
And eventually the bishop/archbishop/patriarch of Rome claimed to be first among equals, the highest of the 5 patriarchs ,because Rome was the highest city in the Empire. And eventually the Patriarchs of Rome claimed to be not only the senior patriarch but to be above the other Patriarchs, and to be the religious equivalent of the secular emperor, a claim not recognized by the other patriarchs.
And the patriarchs of Rome claimed that since St. Peter had been appointed head of Christ's followers, that position must have been handed down to successors of St. Peter - although it would be just as probable that the power would end when Peter died. and since it was claimed that St. Peter came to Rome and was martyred there, St. Peter must have been the first bishop of Rome, and the power of sT. Peter must have passed to St. Peter's successors as Bishop of Rome.
I note that the position of bishop of a city and head of all Christians were two separate and distinct jobs, and that the Christian Bishop of a city was like the secular chief magistrate of the city, and the head of all Christians would be like the secular emperor.
So where was St. Peter when Christ died and St. Peter became the head of all Christians? In Jerusalem. So St. Peter should have become the head of all Christians in Jerusalem at the Same time as he allegedly became the head of all Christians everywhere. So the position of bishop of Jerusalem should have been founded by St. Peter when Christ died, and the later bishops of Jerusalem should have been the successor of St. Peter as head of all the Christian church.
So the whole papal claim to be the head of all Christians seems a lot like confusing two different jobs with each other.
And such confusion of a lesser job and a greater job could possibly apply to confusing a Roman Emperor, ruling all the Empire including Britain, with a Roman client king ruling in all or part of Roman Britain.
During the Roman Republic the Romans hated the kings of the Roman Kingdom, and hated the idea of ever being ruled by kings again. That is why the Roman Emperors were known by titles like princeps, imperator, caesar, augustus, pontifex maximus, dominus, etc.
But in the late Roman empire, after centuries of imperial rule, Roman hatred of the title of rex or king faded a bit.
In occasion of the campaign of Constantine against the Sassanids (337), Hannibalianus was made Rex Regum et Ponticarum Gentium, "King of the Kings and of the Pontic People". Probably it was Constantine's intention to put Hannibalianus on the Pontic throne, after the defeat of the Persians.
So by then the Romans may have gotten over their hatred of (Roman) kingship as well as their fear of Hannibal.
In later centuries the Roman emperor was sometimes refered to as rex, "king". Thus it became possible to speak of an emperor as a rex Romanorum.
Syagrius was a Roman ruling a part of northern Gaul, which modern historians call "the Kingdom of Soissons", until it was conquered by Clovis, king of the Franks in 486. Nobody knows what title Syagrius used, or whether he claimed to be an official of a Roman emperor or and emperor himself. A century later Gregory of Tours described Syagrius as a rex Romanorum "King of the Romans".
Historians have mistrusted the title "Rex Romanorum" that Gregory of Tours gave him [Syagrius], at least as early as Godefroid Kurth, who dismissed it as a gross error in 1893. The common consensus has been to follow Kurth, based on the historical truism that Romans hated kingship from the days of the expulsion of Tarquin the Proud; for example, Syagrius' article in the Prosopography of the Later Roman Empire omits this title, preferring to refer to him as a "Roman ruler (in North Gaul)". However, Steven Fanning has assembled a number of examples of rex being used in a neutral, if not favorable, context, and argues that "the phrase Romanorum rex is not peculiar to Gregory of Tours or to Frankish sources", and that Gregory's usage may indeed show "that they were, or were seen to be, claiming to be Roman emperors."
So in late Roman times an emperor could be described as rex, "king". And if a late Roman source described an Emperor as a rex in relation to Britain, possibly someone even later might think that emperor was a king of Britain.
A number of usurpers wee proclaimed in Britain. In 407 one of them, named Constantine, led most of the army in Britain to Gaul. Constantine took over Gaul and Hispania, and was recognzied as an official emperor, Constantine III. Constantine III was eventually defeated and killed in 411 by Constantius, a general of the legitimate emperor Honorius. Constantius was later briefly emperor Constantius III, just to confuse people.
Procopius in his History of the Wars, written about the 530s, says that since the time of Constantine III the central government lost all control over Britain, and Britain was ruled by tyrants from then to his present time. In that time tyrant was the word for a Roman usurper, who claimed the emperorship but failed to become a legitimate emperor.
So Procopius seems to have said that Britain was ruled by claimants of the imperial title, a line of hypothetical northwestern Roman emperors. And there are other hints for those possible emperors in Britain. And of course nobody knows much about the specific imperial titles they would have used, and possibly they would have often used the title of rex, which might make it possible for those emperors to be mistaken for kings.
St. Gildas, writing in Britain within a few decades before or after Procopius, mentions that Britain had kings, without specifying kings of all of post Roman Britain or kings of various smaller parts of post Roman Britain. Kings of all of post Roman Britain might have been the same thing as emperors of a hypothetical Roman empire in Britain.
Centuries later the Historia Brittonum mentions several post Roman leaders of presumably all the Romano-Britons as kings, and mentions a contemporary ruler as "King of the Britons" presumably meaning king of all the Britons, overlord of all the other British kingdoms.
A number of British & Welsh kings have been called "king of the Britons" over the centuries.
And it is possible that if the hypothetical breakaway Roman emperors in Britain were usually called rex their title might evolve into King of the Britons over time, with rex having a meaning closer to "emperor" than "king".
I also note that there is a tendency to refer to the ruler of a country as the ruler of their capital city, town, or village. So the high king of all Ireland was called the king of Tara, the King of Gwynedd was called the King of Aberffraw, etc. And even in modern times there is a tendency to do the same, speaking of the Caliph of Cordoba, the Caliph of Baghdad, the Emperor of Trebizond, the Emperor of Nicaea, the Latin Emperor of Constantinople, or the Emperor of Rome.
So maybe some of the provincials in Britain called the emperor of the Roman Empire the king of Britain.
Since Roman Britain contained several former British kingdoms, possibly the Roman-Britains called the Roman governor the King of Britain.
As you know, in modern times it is customary to refer to a ruler by his personal name followed by a Roman numeral, his number as a monarch of that personal name ruling his realm.
That is not the custom when mentioning Roman Emperors before the early "Byzantine" period. But I have researched what Roman emperors would be called if that was the custom with them.
Emperor Nero (r. 54–68) was born Lucius Domitianus Ahenobarbus so might be counted as Lucius I.
Galba might be counted as Lucius II.
Lucius Verus (r. 161–169) would be Lucius III, or Lucius I if you don't count Nero or Galba.
Commodus would be Lucius IV, or Lucius II if you don't count Nero or Galba.
Lucius Septimius Severus (r. 193–211) would be Lucius V, or Lucius III if you don't count Nero or Galba.
Caracalla would be Lucius VI if you count him as a Lucius.
Aurelian (270–275) would be Lucius VII.
I note that governors of Roman Britain included:
Lucius Javolenus Priscus (84–86).
Lucius Neratius Marcellus (c.101–c.103)
Lucius Ulpius Marcellus (x.179–c.184).
Lucius Artorius Castus (c.187–c.1910. Acting governor.
Lucius Allenus Senecio (c.205–c.207)
Lucius Septimius was governor of Britannia Prima sometime in the 4th century.
In the Middle Ages it was claimed that St. Helena, wife of Constantius I and mother of Constantine I, was a British princess, the daughter of Old King Cole who was king of or in Britain.
Geoffrey of Monmouth, in his History of the Kings of Britain written about 1135, claimed that Britain was a vassal kingdom of the Roman Empire and not a province. He claimed that Constantius I succeeded his father-in-law Cole as king of Britain and his son and heir Constantine I became king of Britain and invaded and conquered the Roman Empire from Britain.
Geoffrey also claimed that Constantine III, the usurping Roman Emperor who came with an army from Britain, was also king of Britain as Constantine II. So that makes two Roman Emperors, Constantine I and Constantine III, that Geoffrey claimed were also Kings of Britain. And maybe Geoffrey also made Constantius I both Roman emperor and king of Britain.
And possibly the old book that Geoffrey claimed to be his source also claimed that Constantius I, Constantine I, and Constantine III were kings of Britain in addition to, or instead of, being Roman emperors, and so it is possible that some British people did mistake Roman Emperors for kings of Britain.
Here is a link to an article about client kingdoms in Roman Britain.
Note that one of those vassal kingdoms, the Regnenses, was ruled by Tiberius Claudius Cogidubnus. And in an inscription Tiberius Claudius Cogidubnus was described as "great king of the Britons". That implies a claim to rule all of the Britons, even though the Roman government ruled only part of Britain, and Cogidubnus ruled only part of Roman Britain.
So that shows that there could be client kings in Britain who used Roman names, and that exaggerated claims that they ruled all the Britons could be written in stone.
So it is conceivable that there could have been a Roman client king in Britain named Lucius, and it is possible that there could have been exaggerated claims that he was king of Britain instead of only a part of it.
I think that there could have been more kings in Roman Britain than the article suggests. The rich lands of southeastern Britain in what later became England, are full of Roman cities, Roman villas, farms with Roman artifacts, a dense network of Roman roads, Roman forts, etc., giving evidence of Roman rule, and a civilised Romano-British society.
But in Dumonia in the southwest, the later Cornwall and Devon, and in most of Wales in the west, and in the north, evidence of Roman civilian life, is rare, and in many places even Roman military posts are few.
In those places, Celtic tribes would have continued to live much as they did before the Roman conquest, and they may have been ruled by kings, who presumably usually avoided causing serious trouble with the Roman garrisons — the Roman Empire was thousands of times more powerful than any British tribe. So those hypothetical British kings of those tribes would have usually lived in peace with the Romans and acknowledged the overlordship of the Emperor.
Just as many Indian tribes in the American west in the 19th century avoided serious trouble with the Americans and acknowledged the overlordship of the US government and so don't get mentioned much in history books.
So I have mentioned a few ways there could possibly be some slight historic foundation for the story of St. Lucius.
Lucius (full name Lles map Coel) was a legendary 2nd-century King of the Britons and saint traditionally credited with introducing Christianity into Britain. Lucius is first mentioned in a 6th-century version of the Liber Pontificalis, which says that he sent a letter to Pope Eleutherius asking to be made a Christian. The story became widespread after it was repeated in the 8th century by Bede, who added the detail that after Eleutherius granted Lucius' request, the Britons followed their king in conversion and maintained the Christian faith until the Diocletianic Persecution of 303. Later writers expanded the legend, giving accounts of missionary activity under Lucius and attributing to him the foundation of certain churches.
The first mention of Lucius and his letter to Eleutherius is in the Catalogus Felicianus, a version of the Liber Pontificalis created in the 6th century. Why the story appears there has been a matter of debate. In 1868 Arthur West Haddan and William Stubbs suggested that it might have been pious fiction invented to support the efforts of missionaries in Britain in the time of Saint Patrick and Palladius. In 1904 Adolf von Harnack proposed that there had been a scribal error in Liber Pontificalis with ‘Britanio' Britannia being written as an erroneous expansion for 'Britio' Birtha or Britium in what is now Turkey. The full name was 'Britio Edessenorum,' the citadel of Edessa, present day Şanlıurfa in Turkey. The name of the King of Edessa was Lucius Aelius Abgar.
So the story of a King Lucius in or of Britain who converted to Christianity was told by sometime in the 6th century (501-600).
He is supposed to have lived in the time of pope Eleutherius (c.174-189), but if there is a real basis for the story the Lucius in question might have lived earlier or later.
Lucius Aelius Megas Abgar IX was king of Osroene from 179-214.
During the reign of Abgar the Great, Christians were favored in the realm of Osroene. It is thought by some that this led to the story of the letters between Abgar V and Jesus of Nazareth. In 1904, Adolf von Harnack proposed that Abgar IX may have been the origin of the story in Liber Pontificalis that King Lucius of Britain wrote to Pope Eleuterus.
The now orthodox view[clarification needed] of Lucius has been challenged by Southampton University educated archaeologist David J. Knight in his book 'King Lucius of Britain', where Knight suggests that Abgar was King of Edessa, not ‘Britio’, which was the local name for a castle within his realm, Knight suggests that he is never called Lucius of Britio/Birtha in contemporary sources, only Abgar of Edessa. Knight argues for accepting the ancient tradition, that the Lucius who wrote to Pope Eleutherius was a British ruler.
I have suggested a few ways that the idea that there was a king named lucius in Roman Britain could have been started.
I note that few Roman Emperors visiting Britain, or Roman governors in Britain, even if named Lucius, would have been interested in a little known and sometimes persecuted religion like Christianity.
Certainly Christian missionaries did visit Britain and convert some people some time before Constantine I did convert to Christianity.
And it is possible that a Roman client king in Britain could have been named Lucius and be interested in Christianity. However, the Roman client kings in Britain would rule tribes in the backward out of the way parts of Roman Britain, and would seem rather unlikely to have Roman names or to meet Christian missionaries in the cities and towns.
But the royal families in pre Roman Britain probably intermarried with each other, often marrying princesses from hundreds of miles away, and that may have continued after the Roman conquest, with kings in the wild highlands marrying princesses of deposed royal families in the rich lowlands, and sometimes inheriting property from their relatives there.
So some client kings in Roman Britain may have spent part of their time ruling their highland tribes, and part of their time relaxing in villas and towns in the lowlands, using Roman names while in the lowlands and having chances to meet Christian missionaries.