The British (wealh) in Hild are definitely British and not Romano-British; the Roman influences I mentioned come from priests following in Augustine's footsteps to reconvert Britain to Christianity, and are presented as being in conflict with the surviving Celto-British Christians in western areas.

I think Griffiths has done a huge amount of research, though I did see a blog post where she said that she normally stops researching at the point where she starts writing, so she may well have been Jossed by more recent discoveries.