The British of the post-Roman period occupied Cornwall, Wales, Scotland south of the Highlands, and well into modern England. During the fifth century they would lose ground consistently.
The British were at least the second invasion of Celts to come to Britain. They came sometime after the eighth century before the Common Era and spread to every part of Britain south of the Highlands. Most of them were conquered by the Romans in the first century of the Common Era and would incorporate Roman culture into their own, resulting in some beautiful artwork.