I’m going to say that it’s punishment by the process. Britain wanted to be a separate nation, so by golly, it will be treated as a separate nation and all t’s will be crossed and i’s dotted to ensure that it would be treated as a different nation.
Britain isn't one nation - "Britain" by that I guess you mean "The United Kingdom of Great Britain and Northern Ireland" consists of 4 nations: England, Scotland, Wales, and Northern Ireland.
The UK is a separate country and that has never changed - for the same measure Hungary is a separate country.
The EU federalists want a closer union where that statement "separate country" could change if they get their way, but it isn't there yet