Where all this gets interesting, for me at least, is what the argument about the real Arthur says about history as a discipline and a peculiar paradox at its core.
History, as an academic discipline, is focused on “the evidence”. For someone interested in the early middle ages that means source analysis becomes an obligatory part of the activity. But that source analysis quickly disappears into numerous areas of specialisation, each with its own arcane rules and practices. Of course if you’re suffering from “we don’t need experts” you can ignore all this and crash around in your own preferred version of the past, believing whatever suits you.
However, whichever discipline of history you subscribe to and practise limits what you can see. A good historian is aware of this. An archaeologist who wasn’t wedded to a fifty minute TV spot and the possibility of a lucrative book tie in would remember that were he or she to excavate the site of the Crucifixion of Jesus of Nazareth, the “evidence” would be three stains in the ground to show the post holes. Nothing in the excavation would be able to explain the significance of that site or the impact it had on world history. (Which makes you wonder what Francis Pryor thought he was doing writing “Britain AD: In Search of King Arthur.” Which is a silly book.)
However (number two) focused on "Evidence", it's easy to forget that any document is one moment of stability nailing down the swirling ebb and flow of rumour and fear and guesswork and ignorance. Everybody knows that not everything gets recorded even now; how much less in the fifth and six centuries. Because it wasn't written down doesn't mean it didn't happen. And everybody knows that not all the evidence survived and everybody knows that survival is no guarantee of the quality or importance of the evidence.
Two examples: when the Anglo-Saxon Chronicle records the arrival of the first ships of the Northmen in 797, all that entry means is that this is the first arrival that has been noted and remembered and brought to the awareness of the writer. To give another example, we know the Romans in Britain were using Germanic mercenaries long before 450 AD, the traditional date of their arrival, but that doesn't mean someone didn't call in three ship loads led by a man called Horse who turned on his employer.
If we can't find Arthur in the remaining evidence, the irony is that it doesn't mean he didn't exist.
Because the oddity, or the paradox, is that if there is no evidence to prove Arthur existed; there is no surviving evidence can prove he didn’t. And while in historical terms the burden of proof has to be on the people asserting that he did, in that wonderful little gap marked by the semi colon, the creative imagination is free to ramble.