Very roughly, Britons were the main population in southern Great Britain (roughly England, Wales and Scotland to the Firth of Forth), but were "squeezed" during

Very roughly, Britons were the main population in southern Great Britain (roughly England, Wales and Scotland to the Firth of Forth), but were "squeezed" during the Anglo-Saxon settlement into more isolated pockets — Cumbria, Wales, Cornwall — and some crossed over to Brittany. How much of of the "squeeze" was population replacement as opposed to languistic and cultural change is up for debate.