The Romans, came, they saw and they conquered. But they left no genetic imprint. The same goes for the Vikings and the Normans, with the exception of the Orkney Islands, which were Norwegian for 600 years. Nevertheless Viking DNA only accounts for 25% of today’s Orcadian DNA.
The New Scientist reports on DNA mapping undertaken of Britain’s Caucasian population. To qualify for sampling people had to have all four grandparents born within 80 kilometres of each other. So I guess there’d be a bias towards clustering, and clustering is what they got.
There were two main findings. First, the only invaders to leave a distinct genetic genetic footprint were the Anglo-Saxons in the east and the south, who arrived from AD 450. Still the DNA of earlier settlers dominates, with at least 60% deriving from earlier immigrants.
Secondly, there is considerable variety in the DNA of those earlier settlers, especially in the west and the north. In the south and the east the effect of the Romans was to dissolve somewhat the tribal clustering, partly through building roads.
The first immigrants arrived from about 9000 BC, as the ice melted, via land bridge from Belgium and Germany and by boat from France. The overall pattern is captured in this image:
The article does not mention language.
English, of course, is a Germanic language which came with the Anglo-Saxons. German as a language only developed from about 500-0 BC in what is now southern Scandinavia. The Germans emerged from the north onto the main European continent and beyond from about 0 AD. The Celts were already there. But the Celtic language only dates from about 1350-850 BC and appears to have arisen in central Europe. It’s part of the Indo-European family, which arose far away in the steppes north of the Black Sea, probably around 4000 to 3000 BC.
Quite obviously the early settlers of Britain spoke something else. But the Celtic language when it came took over to become the language of the masses. Whether this was a matter of sheer numbers, or cultural influence I don’t know. How this all came to be goes beyond my pathetic knowledge of ancient history.