Well, but don't forget this "apartheid" theory is a model trying to account for the dominance of genetic heritage and "known" numbers of emmigrants/conquerers. If next week new evidence revises the numbers of either indigenous population or conquerers, the model goes out the window. Nothing against P. J. Heather, who I've not read, but how good is the documentation of population movements in 400 ad? I've no idea.

But the point is that the indigenous people WERE displaced en masse, albeit over fifteen or twenty generations during which there was NOT a substantial amount of intermarriage. This model says it happened because the natives were kept in poverty and servitude; their kids consistently died earlier and more often until most of the people in the Anglo counties were Anglo, not Welisc. What kind of "respect" do you imagine the Anglos had for the Welisc religion(s)? Of course the Anglos had to learn a few words of Welisc here and there, if only to give orders.

In the larger population centers, there would have been some number of "dark Celts" dying their hair blond, angling for an Anglo spouse and trying to join the Church of Wotan. Just the way some Britons became Romanized four centuries earlier. In the hinterlands, old wives would have been leaving tat by the wells as their mothers had done forever. Their peasant farmer husbands wouldn't have cared less which gods were in charge, so long as the animals and crops were left in peace. Their numbers were dwindling, though.

Eventually, the two groups became so accustomed to each other they began to intermarry and their customs and beliefs were all tangled up. From our modern vantage, though, how different had they ever been? Were Bran and Wotan recognizable cognates the way Jupiter and Zeus were?

So much history, all muddled up!