First, have you read the two Osprey books, by Simon MacDowall? They are Late Roman Cavalryman 236-565AD
and Late Roman Infantryman AD 236-565
, both available from Amazon.
It is my opinion that Arthur's knights were more probably an evolution of the Sarmatian cataphracti
sent there by the Emperor Marcus Aurelius around 270 CE or so. He had hounded the nomadic Sarmatians in a winter campaign very similar to the one in the winter of 1876-1877 that broke the Sioux after the Battle of the Little Big Horn and they finally agreed to a treaty and part of that was that they would supply him with a regiment of their heavy armored cavalry. He sent them to Britannia to serve out of Eboracum (York) as a mobile force along the Wall. Now, they had largely been absorbed by the locals by the middle of the 4th Century when the Barbarian Conspiracy* occurred bringing the Master of Horse, Count Theodosius**, into Britannia in 368 CE with a Field Army to sort things out. He reorganized things and then left behind a reasonably solid defense that lasted until the garrisons were again stripped by the pretender to the throne, Magnus Maximus, when he sailed for Gaul in 383 to make his play for the imperial purple. The garrison never really recovered from that and, by 410, the Emperor Valentinian sent the Britons word that they must look to their own defenses, that Rome could no longer defend them.
During the period following the collapse of Roman authority in the West, it was the practise of a warlord to have a group of bully-boys swear oaths of loyalty to him and they were frequently called his "companions" much as Alexander the Great called his personal guard troops his "Companions." The Welsh word for that is "cambrogi" and that is what I envision the local warlord Arthur as using, a group of trained professional heavy cavalry in the cataphract mold who had sworn oaths of loyalty to him. In the case of Arthur, I would posit that he tried to maintain a level of discipline that prevented his "cambrogi" from becoming simple bully boys and running wild over the locals and that is why he is remembered in a favorable light.
As a matter of usage, generally the term "cataphracti" is applied to the heavy armored cavalry as used in the West and the term "clibanarii" is applied those in the East. There seemed to have been some differences with those in the West following the Sarmatian pattern more than the steppe archer pattern of the Parthian/Persian clibanarii. BTW, the term "clibanus" is Latin for "oven" and the koinee Greek for it is "klibanion." If you ask me, that name must have been applied to the clibanarii as a result of wearing full armor in the desert heat.
and, from Ammianus Marcellinus: http://www.dot-domesday.me.uk/barbarian.htm
** Father of the Emperor Theodosius who actually went to Britannia with his father in 368.