> We have no reliable evidence regarding what proportion of Roman Britain was "black". We have good historical evidence that there were Romans present in Britain who we'd these days describe as black (e.g. an Ethiopian soldier garrisoning Hadrian's wall).

That "evidence" consist of one fleeting mention in a biography about Septimius Severus.

> For 101 reasons, the genetic evidence is not very informative, and certainly can't be used to justify that claim that Africans were a "statistically insignificant subgroup"

The lack of evidence (genetic or historical) puts an upper bound on the proportion of subsaharan Africans in Roman Britain that is several orders of magnitude lower than the BBC animated cartoon suggests.

> As Beard points out, even Norman DNA is not very well represented in the modern population, and we know for sure that plenty of Normans came over as a result of the Norman invasion.

For the presence of the Normans there is plenty of historical evidence. For the presence of subsaharan Africans there is just about nothing. So we should stick to the null hypothesis.