Kingdom of Great Britain is a Kingdom. Great Britain without the Kingdom part is a geographical location. It's like calling USA America. Unofficial moniker is plain wrong to use in an official speech or a grand strategy videogame.
It *was* a Kingdom. As it currently stands the British monarch is not the monarch of Great Britain, and also the monarch of Northern Ireland. They are the monarch of the United Kingdom (singular) of Great Britain and Northern Ireland. So Great Britain is now simply a geographical reference to a component of the United Kingdom (again singular).
The original point stands the official title of states can and do change quite rapidly reflecting cultural changes or simple propaganda by the party currently in power. Try find Burma on an up to date map. Or Rhodesia. Or try find the city of Bombay. I don't see what anyone gains by arguing against that reality.
In either case, I think the original ask is very much on the edge of "Dont care", and as some have noted its possible to custom name any CK 2 title in vanilla so its possible (though not important) that Stellaris would support such customisation for whoever cares to update their faction name for whatever reason.