No, they have not. Roman Briton was a different society than pre-Roman Briton. The Anglo-Saxons came about 1500 years ago and nearly completely replaced the Britons, genetically and culturally. The Norman invasion was about 950 years ago and greatly changed Anglo-Saxon culture, although not as much as the AS changed post-Roman Briton. There were several other invaders (Danes, etc.) as well.

By contrast, the Japanese are perhaps 3,000 years old as a distinct ethnic group on the island.