England is one country of the United Kingdom. Scotland, England and Wales make up the island of Great Britain, the largest of the British Isles. The UK also includes Northern Ireland. Hence: The United Kingdom of Great Britain and Northern Island.
I was confused too, thought that Great Britain referred to the archipelago as a whole (excluding Ireland) rather than the largest island.
For a long time I've felt that one of the traditional titles of the Russian czars, "Czar of all the Russias", was cooler than the titles I was aware of for other rulers. I was surprised to find the seal of an English queen on Wikipedia which clearly showed a Latin title translating as "Queen of all the Britains".
The switch from "King of Britain" to "King of the Britains" seems to have occurred during the reign of George III, judging by https://en.wikipedia.org/wiki/Great_Seal_of_the_Realm . None of those include an "all", but I'm sure it was present in the seal I saw.