That's what I was - rather shamefacedly - saying. Sutcliff (thanks for the correction!) writes in almost every one of her novels about this strata of British society. We do act (at least, I do) like the Celts were the primary ethnic mix, when this is, of course, not so - the Celts were conquerors, driving the former inhabitants back into Wales, Scotland, and elsewhere.

A quick Google search gives me tons on the pre-Roman Britons, but it's always about the Celts!

One thing I do dimly believe . . . that there are racial and religious connections between Sutcliff's "little dark people' and the Picts, but I'm not even sure of that!