From 1776: John Adams, talking to Benjamin Franklin:
It doesn’t matter. I won’t be in the history books anyway, only you. Franklin did this and Franklin did that and Franklin did some other damn thing. Franklin smote the ground and out sprang George Washington, fully grown and on his horse. Franklin then electrified him with his miraculous lightning rod and the three of them - Franklin, Washington, and the horse - conducted the entire revolution by themselves.
Sounds like Adams speaking from his experience as a lawyer giving summations before a jury. There couldn’t have been much money in being a professional heckler in the 18th Century.
If you mean the Chequers Estate Act of 1917, it didn’t create the office of Prime Minister; it provided a residence for the “official now popularly known as Prime Minister.” The existence of the office of Prime Minister is taken as given, without any attempt to define it.
In The Adams Chronicles, which was a remarkably accurate dramatization of history (much of it lifted directly from the writings of the Adamses themselves), George III tells John when he’s presented as US Ambassador to Great Britain, “Mr Adams, I was the last to agree to separation.” (I believe those were the exact words that were used.)
If George wasn’t the driving force behind suppression of the rebellion, who was? Lord North?
I am forever amazed at how informal the UK’s Constitution actually is. One of the most important offices just kind of happened and was never formally defined. It seems similar to the Privy Council (of which Canada has its own, as I understand) which is not to be confused with what transpired between two companions of the bath, which would be more accurately called the Crapper Colloquy.
The office of Canadian Prime Minister is similarly undefined. There were only two references to the Prime Minister in the Constitution of Canada, in two provisions which have now been repealed. There is currently no reference to the Prime Minister in the Constitution.
British constitutional “law” consists of more than just statutes; it includes a wide range of unwritten conventions which, even though they’re not codified, are still complied with in political practice and considered part of the “law”. There surely was such an unwritten convention long before 1917 according to which there was a political office called Prime Minister, appointed by the monarch but accountable to Parliament, who would have the power to advise the monarch on a lot of things, including ministerial appointments.
And getting back to the question from the OP, note that the office came into its modern form in the 1830s. During George III’s reign, there was a Prime Minister, but the King still took a more active role in the government than monarchs from Victoria onwards. I would be surprised if the tradition of the King’s Speech being purely the product of the ministry of the day had emerged during George’s time; I would think he had some input into it.
I’m not saying the office did not exist. I am saying that “Prime Minister” was not an official title before 1917, when it began to appear in statutory instruments.