Excerpts from Books and Wikipedia
"In 485, just as Clovis was beginning his decade of conquest, Ambrosius Aurelianus led the rulers of Britain in a great battle against the Saxon invaders. For thirty years, the native British had been fighting against this invasion. Keeping the Saxons and Angles away had become a way of life for two generations. Vortigern, who had managed to kill one of the original Saxon generals back in 455, had led the resistance for another fifteen years before dying, with the enemy still on his doorstep. . . . At Vortigern’s death, Ambrosius had taken up the position of chief British warlord. . . . But “native Britain” was weak, welded together as it was only by the necessity of keeping Saxons out. There was no British kingdom and no British high king, no shared religion, no idea of nationhood. . . . Ambrosius became immortal as “Arthur, King of the Britons,” a man fighting for civilization and order in a world where neither was natural: always threatened by the power of other warleaders, brought down at the end of his life by the treachery of those he should have been able to trust." [Bauer: Medieval World, p. 175-7]
Between 572 and 604, the pope sent missionaries to Britain. [Bauer: Medieval World, p. 258-9]