Let's create a Romanno British King Arthur, based on the original idea, first I gonna write some rules on the OP, and then we can discuss if it is possible or not.
1- Arturius, Arthur, Artor of whatever we call him, must be a roman-british, as in the legend. He also needs to be christian.
2- This figure must be born around the same time as the OTL legend, on the IV or V century, and must fight the saxon invasions.
3- Such character needs to unite the romano british states behind himself, gaining some variation of the title of King of England, be it Rex of Britannia, or a more celtic like title of King of Britons.
4- He must stabilish a dynasty, the dynasty doesn't need to last long.
5- This Kingdom doesn't need to unify all britannia, but at least rule over all the post roman britain south of hadrian wall.
6- There is no need for a "round table", but this kingdom must be feudal, not a bureocratic one like the roman empire, neither a centralized empire as the eastern roman empire.
What do you think, it is possible for such a character to rise and make such scenario work, a real King Arthur?