If you had the Romano-British left after Rome abandoned Britain actually manage to halt the Anglo-Saxon conquests, I think Britain would be far more isolated from the rest of Europe with far more focus on their own business (rather than attempts to get involved in France etc). All the parts of Britain would probably be drawn together faster and will less bloodshed than OTL, as the various cultures would have far more in common, being mostly Celtic with a Roman influence which would decrease with time. If I recall correctly, the Britons tended to be split that way in OTL after Rome left, half wanting to go their own way and the other half looking to Rome.