Ah good point, but the scenario in question involved both Ireland and Scotland leaving so "Great Britain" doesn't fit either.

If both leave, then the predecessor to the Kingdom of Great Britain was the Kingdom of England

Wales was already part of England, as principality / having been conquered

Though "United Kingdom of England" could argue that they kept the Irish crown :)

I feel like "Kingdom of England" would be off the table - you probably wouldn't want to antagonise the Welsh immediately after they just watched NI and Scotland split off :)