Interesting theory about the origin of the Arthur legend.
:^) Based on post-Roman large structures (so-called Offa's Dyke is hundreds of years before Offa; the Wansdyke was dubbed that -- 'Woden's Dyke' -- by the A-S because they couldn't believe it had been built by humans), there was some kind of post-Roman authority prior to the Anglo-Saxon dominions, but I think the Arthur legend (other than the most familiar one, which grew entirely in the mind of Sir Thomas Malory and was very much of its time and influenced by unrelated French tales) was a pastiche of different legends that reach back into the Roman occupation era, and perhaps before. Of what I've read (which isn't much), Geoffrey Ashe's book "King Arthur: the True Story" was most compelling. The most fun fictional version is the Mary Stewart series, and that author includes an essay in one of the volumes discussing the possible historical foundations.