I view Arthur as a composite character, repurposed for different agendas at different times. If there were surviving pre-Roman sources, I’d not be surprised to see him there. No telling when the name started.
Here’s an old FReepmail I sent to someone in April of last year:
There must have been some kind of patchwork of post-Roman kingdoms of which very little remains in contemporary writing. My guess is, the Roman Hadrian’s Wall, and the two “dark age” earthworks, so-called Offa’s Dyke (it’s certainly much older than Offa), and the Wansdyke, probably represent the north, west, and south borders of some political entity.
The “Arthur” that wound up being part of childhood literature in the English-speaking world is a recent invention. The name Camelot is certainly a corruption of Camulodunum, modern Colchester, and may or may not have been its capital, but clearly such a naked coastline must have been important to such a post-Roman state.
[Alternatively,] The Car Dyke, the 80+ mile Roman-era canal, may have been the eastern border, however.
the period of transition from romano-british rule to conquest by various saxon groups is definitely hard to nail down. what I recall that we have some level of documentary or archaeological indication of is that a kingdom in.around cumbria (thus the name), wales, and cornwales were all that ultimately survived of the p=celtic peoples in the roman areas. (the picts are another matter).
obvious there was some gradual (or not) process of the rest of what would be england getting assimilated/conquered by germanic tribes, and surely there was time for a few battles to be won, buying time (perhaps years or decades) for an area.
i wish we knew more.