All of this basically hinges, I think, on the Britons being able to hold the Severn Estuary (Gloucestershire, basically). Hold the Severn, and Cornwall and Wales will remain united, and relatively strong; lose it, and the West Country alone cannot really hope to hold back the English. Lose the Severn, and Wales loses one of its major natural defences, and as a result, will be forced into the Welsh hills. Of course, if that happens, I think that Wales and Cornwall are much less likely to divide, and they will probably never diverge as significantly as OTL.