I remember being taught about this when I studied the Stuarts at college. As I recall, the phrase:
" ... ancient Laws and Constitutions at this time unknowne."
meant laws that were no longer recognised in English Law. Now, given the state of many of the public records at that time (many of which had not been stored in ideal conditions!), it is probably also true to say that many of the ancient laws were, indeed, unknown. However, "Unknowne" in this context is simply a synonym for "not recognised".
[The National Archives have produced a podcast that describes the history of the Public Records Office, which includes some information about how and where the records were stored in the past. I'm sometimes surprised that anything has survived at all!]
I do know that some of the earlier oaths survive. The earliest is the oath composed by Archbishop Dunstan for the coronation of Edgar 973 AD:
Three things I promise in Christ's name to the Christian people
subject to me:
First, that the church of God and the whole Christian people shall
have true peace at all time by our judgement;
Second, that I will
forbid extortion and all kinds of wrong-doing to all orders of men;
Third, that I will enjoin equity and mercy in all judgements, so that
God, who is kind and merciful, may vouchsafe his mercy to me and to
you.
The oath sworn by Edward II in 1307 included the catechism from the Archbishop:
Sire, will you grant an keep and by your oath confirm to the people of
England the Laws and customs granted to them by the ancient kings of
England, your righteous and godly predecessors, and especially the
laws, customs and privileges granted to the clergy and people by the
glorious King Edward, your predecessor?
to which the King replied:
I grant and promise.
This is the earliest reference that we know of to "Laws and customs granted to them by the ancient kings of England". A similar reference appears in the text of the coronation oath of Edward IV from 1461, which has also survived.
The Coronation Oath sworn by Henry VIII still survives in the collection of the British Library. It still bears the annotations of the king himself. A digital copy can be viewed online. However, we cannot say for certain whether Henry actually swore the original version of the oath, or the amended version at his coronation in 1509.
A translation and transcription of Henry VIII's Coronation Oath is also available online (although this does rather take away some of the fun of deciphering his handwriting in the original).
As you can see, this again retains the reference to
"... the laws and customs given to them by the previous just and
god-fearing kings."
This was used as the basis for the coronation oaths of the subsequent Tudor monarchs, with amendments to the parts of the oath that referred to the Church, according to the religious preferences of the monarch. For example, Thomas Cranmer amended the oath taken by King Edward VI in 1547, so that:
“reformation of the Church could now be enabled by royal prerogative,
the king as lawmaker”
The Stuart kings James I and Charles I both kept the form of the coronation oath they had inherited from the Tudors, and after the Restoration, Charles II and James II both took the same oath as Charles I.
Following the Glorious Revolution, however, Parliament felt able to flex its muscles. This led to a series of acts that redefined the English monarchy, beginning with re-writing the coronation oath, set out in the Coronation Oath Act of 1688, and ending with the Bill of Rights, agreed by William and Mary in 1689.
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