The United Kingdom Of Great Britain and Northern Ireland.

Still holds if the petulant dreamers in skirts bugger off.

Just because not all of Great Britan would be in The United Kingdom, there would still be a kingdom within it, and about as united as it has ever been...

In any case, until the citizens of The UK are asked, yoh can't actually break it up. Bits can leave it, as there is an outside chance of happening for Scotland, but The UK exists and shall continue to exist regardless.