Who gets Wales?
Seems like the obvious fulcum in a three-way split of the British Isles is where does a recognizable "Welsh" state go - otherwise the amount of arable land is just overwhelmingly in the "English" favor.
So how about this:
Ireland and Wales are united after the collapse of Roman Britain into a recognizable "Romano-Gael" syncretic state because of the threat of the Germanic invaders; that alone gives Ireland & Wales a better "starting point" in the Medieval era and since, andincorporating the Isle of Man gives the resulting "modern" state more resources than the sum of its respective RL parts today;
At a later date, the Norse/Norwegians unite with a recognizable "Celtic" Scottish state in the Medieval era; that state grows to incorporate Northrumbia, the Faeroes, and Iceland; same result as above;
In roughly the same period, an "England" dominated (more or less along historical lines) by Germanic peoples is fused with a Norman kingdom that retain substantial holdings on the European mainland (presumably Armorica/Normandy, perhaps Aquitaine/Gascony) that both improves the position of England alone, but also gives the English a continental focus that leaves room in the British Isles for Scottish-Scandian and Cambr-Hibernian states to survive and grow.
With a three-way split in the British Isles, the obvious directions for expansion of any of the three states are to the northwest for Scotscandia; to the south (the Atlantic Islands/Macaronesia) for the Cambernians; and to the east for Anglo-Normans.
The Anglo-Normans have some tough competitors on the continent; the maritime aspect of the situation for the Scotscandians and the Cambernians means they are likely to dominate the North Atlantic, with all that means.
It's a stretch, but not impossible.
Best,