[smcl]

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

[synotna]

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 :)

[smcl]

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 :)