V century impression

[Franklin]

particularly in a British setting where folks were much more isolated after 410 AD

That's a myth. Britain was no more isolated in 411 than it was in 409.

Oh, you don't have to convince me! I wrote more than one paper at university arguing for the continuity of "Romanized" lifestyles in Britain well into the 5th cent. But there's no denying that each successive generation after 410 became more and more assimilated into Celtic and Anglo-Saxon culture.