Wouldn’t it just be called Britain?
You think Scotland puts the Great in Great Britain ?
‘Britain’ is just the name of the landmass- the island that England, Wales and Scotland sits on, how the population on that piece of map group themselves or divide themselves doesn’t matter, the piece of land is still called Britain. The ‘Great’ doesn’t mean ‘Fab’ it just means Greater as in the larger amount – the island of Britain and its associated surrounding smaller islands (but thats not to confuse it with the ‘British Isles’ which is the whole archipelago and include Ireland and all Ireland’s islands). So it would take the physical removal of those islands to take the ‘Great’ away from ‘Great Britain’
Historically the Island of Britain was know as Greater Britain to distinguish it either from the island next door, (according to Ptelomy Ireland was ‘Little Britain’) and from Britanny, known at times as ‘Lesser Britain’