BTW, that's according to Galician histories in Northern Spain. The English have a different take on the matter but who are they? They lived in a great big old sinking bog-land in Europe at the time!
The people you refer to make up less than 5% of the gene pool.
A very interesting read, if it’s your thing.
So, there are the proto-English ~ living in the Swamps in 700 BC ~ beating each other over the head, swinging bronze swords, drowning victims ~ all the good stuff.
1200 years later they were a bit more advanced ~ then King Ad gave away the farm ~ OUR FARM ~ not his own farm, but ours.
Within about 100 years they too were Christianized and were much less nasty. In fact, they took over leadership of THE CHURCH and managed to spread their own language(s) into a dominant position that wasn't challenged until 1066!
Once the cultural conquest was made all the bitter clingers who demanded their own rites in their own versions of latin were readily turned into what became Englishmen. The real Britons lived on in Ireland, Alba, Brittany, Manx, etc.
True enough the genes are only little removed from each other, but there are always two different versions of the history! Ours and theirs. Ours is better ~ and much more entertaining really.