Either way, that man rallied the people in those dark times, and as long as he lived was a champion of the civilised Britons.
Champion my arse. He held fleeting power compared to his predecessors (Vortigern and Coel Hen), did a naff job at rallying the people, and only fought a successful battle while in flight. Urien, my friend! There was a king and warlord with some balls, genocidal maniac though he may have been. The myth of Arthur is, frankly, a blight on that period of Britain's history. We know nothing of Arthur, ergo, we're supposed know nothing of the isle. Genuine "heroes" and more rightly chronicled figures are sidelined for the sake of this glorified none-entity.

Anyhow, logically looking at it, the Saxons had light armour or no armour, and were more or less infantry. Against such a force, a small contingent of Roman cavalry (who were armoured) could do wonders.
I was under the impression that nigh on all armies fought in more-or-less the same fashion throughout the Migration Era. And moreover that the Anglo-Saxons had been serving as mercenaries in Britian and perhaps other theatres for at least a century by the time of the first rebellions/invasions. Nah. I think in terms of martial technology and tactics there'd have been very little between the Britons and their former hirelings.

That the invaders were defeted at Mount Badon, and that their flow into britain was halted for atleast half a centuary, is a fact. history tells us so. The chronicles of Gildas aid us in pinpointing the time of the battle to the exact decade, and those of Bede throw a little more light on the man who led them.
Their "flow"? I find it hard to believe it could be called such by the time of Badon. Welsh land to conquer was simply running out. For the first English arrivals the only real opponent were the Britons: expanding their realms at the expense of the Wealh. Generations that followed found their growing borders rubbing up to one another. Their old enemies had been pushed to their farthest flung frontiers, on land that held questionable worth, while their new enemies stood poised to take everything they and their ancestors had fought for.

All this said, this subject is very exciting indeed. There looms out of dim pages of history, the legend of Arthur, hazy, yet glittering and intriguing.
To return to an earlier point.. more's the pity.