Not all wales - some Cornwall, some Brittany (hence lesser Britain and Great Britain). For those left behind, no doubt something similar happened like the Norman Conquest where a Saxon aristocracy ruled over a British "peasantry" (not the appropriate word to use, but it'll do). This is not my area of expertise, but my understanding is there are just too few sources from the time to really tell us anything. The only source that comes to mind is Gildas who really just laments the Conquest, saying it's the Britons fault for not converting the Saxons to Christianity (and darn little else)
Rich in history! My daughters went to Welsh schools, and the Welsh language was compulsory learning. I can speak a few words, but it is cool that they are reviving one of the oldest languages of Europe... yet there are less than 600,000 speakers in the world.