I’d like to comment here but haven’t time to do it properly. However, a few points:
The survival of the writings of Gildas and Patrick proves that there was continuity of Latin literacy in post-Roman Britain, so there may well have been other documents, now lost, available to later writers.
As a result I’m inclined to accept the “early” dates (c. 425) for Vortigern and the Saxon adventus given in Historia Brittonum chapter 66, which I think only occurs in a few recensions of the document.* I think they match better with archaeological and continental evidence such as the dates in the Gallic chronicles.
*Also I believe bound with the Annales Cambriae, which includes dates for Badon and Camlann.
If this also means accepting a date of c. 437 for a battle featuring Ambrosius (“Wallop”), this probably makes him too early to be the victor of Mount Badon.
The distribution of “Saxon” inhumation cemeteries in south-east England suggests that they were foederati positioned to resist a re-invasion by Rome. The fates of previous rulers of an independent Britannia from Carausius onwards would make whoever was in charge nervous, this is probably the context for the appeal to Aetius* (active in Gaul c. 425-430) and the battle of Wallop, with dissent between British factions over whether to invite the Romans back.
*This is conventionally dated to his third consulship, but as Gildas misspells his name, he probably didn’t have a written source and merely knew there had been an appeal to a Roman general who had been a consul three times.
Still, Gildas tells us there was a “victor of Mount Badon”, and he seems to think the battle was highly important. Perhaps that victor was called “Arthur”, however, perhaps we should just call him “Vomb”.
In the context of a late 5th or very early 6th century Badon and the archaeology of Anglo-Saxon (an anachronistic term) settlement, the Arthurian battle list in Historia Brittonum makes a good deal of sense, with those that can be (slightly) reliably identified clustering around Lindsey and the Humber estuary or just north of the wall, and other possible identifications elsewhere in eastern England. If Badon was near Liddington hill fort, let alone Bath, it’s a bit of an outlier if anything.
There are good arguments that the “city of the legions” referred to by Gildas as cut off by the barbarians was York, not Caerleon or Chester, as it’s difficult to see how the latter two cities would not have been reachable from wherever Gildas was writing when he wrote, presumably in the first half of the 6th century. In that case the similarly named battle in the battle list would also be at York, again a much more plausible location on the western edge of the Angle settlements in proto-Deira.
Another point that has occurred to me is that the Germanic polities in southern England, which I believe originated from foederati, self-identified as “Saxons” (the term used for them by their British employers), while those further north in East Anglia, Lindsey and Deira, which I believe were founded by groups translocating from the continent en-masse due to sea level change, self-identified as “Angles”.
Following on from this, the “Saxon” settlers on the upper Thames near Oxford, who eventually became part of Wessex, were known as the Gewisse, a term implying something like “the reliable ones”. They could hardly have settled where they did, right in the middle of Britannia and at a very early date (<450), without British approval, and perhaps remained longest as loyal foederati. It’s fascinating to think that if Arthur existed, part of his forces may have been Saxon auxiliaries. It might help to explain the rather ambivalent attitude shown to him in some of the Welsh legends.
Since David Dumville put a great deal of effort into debunking Morris (who admittedly tried to write a narrative history of dark age Britain by incorporating every possible pseudo-historical source), Arthur has become a pretty toxic subject for professional historians and archaeologists. Some of the better material out there comes from competent amateurs, though it all needs to be approached sceptically.
Anyway, written far more than I meant to on my phone. Might try to re-do it properly with references. I also have a map I’ve been working on, but I’m a bit worried it might violate copyrights as it incorporates elements copied off the net or scanned from books.
Slight edit despite all the proof-reading: Gildas of course only says that there was a battle of Badon, which was a British victory. This does of course imply an important British leader and victor - “Vomb”, or maybe “Arthur”.