George III created his own majorities in Parliament, through adroit distribution of money, honors, and patronage. He was not one to sit passively and let Parliament come to him.
Given this fact, and the fact that George III filled volumes with correspondence with his ministers on every conceivable subject, I’d be shocked–nay, stunned–if he never read the Declaration.
But, I can’t prove that he did. There is an odd gap in his correspondence from the summer of 1776. (The Declaration of Independence reached England about August 10.) Conspiracy theorists, have at it.
And I am saying that in a system where not only statutes but also unwritten conventions are accepted as sources of constitutional law, the existence of the office of Prime Minister, together with this title, in such conventions makes it just as “official” as it would be if the term appeared in statute. The British constitution consists of more than just statutory instruments.
Perhaps more to the point: The UK is (mostly?) a Common Law system just like most of America (except Louisiana) and most of Canada (except Quebec) so the basis of law is the same there as it is in most of here; they just sometimes have occasion to cite to cases and other sources of law which are older than the ones our legal professionals use. As near as I can tell, however, the kind of legal reasoning is much the same.
(Also, they have a split profession, with barristers and solicitors, and they use white powdered wigs more often. I don’t understand that nearly as much.)
I have a photo from Westminster Abbey, of the throne showing people’s names carved in it, graffiti style, from an earlier period. Now it has an actual rope around it! (There is probably a bit more surveillance of the spot–particularly following the 1950 theft of the Stone of Scone–but to the tourists wandering through, it appears a bit open.)
The common law of the UK as established before 1776 is still valid in the US*. I don’t know how often it’s used, but it’s available should it be needed.
(* Except for the stuff that’s been explicitly superseded or repealed and so on.)
Oh, there’s an Order in Council appointing him as a minister of the Crown. It’s just that there is no constitutional definition. Of the office nor its powers and duties.
From one perspective as well, the UK Prime Minister’s official job, ‘First Lord of the Treasury’, isn’t even one job but part of a job.
Since 1714, the office of Lord High Treasurer has been placed ‘in commission’, which means it’s shared among a group of people. The Second Lord is the Chancellor of the Exchequer, the UK’s actual finance minister/treasurer.
The Third, Fourth, Fifth and Sixth Lords of the Treasury, the ‘junior lords’, are government whips in the Commons. They are managed by the ‘chief whip’, who himself has the sinecure of ‘Parliamentary Secretary to the Treasury’, also called the ‘Patronage Secretary’