Really excellent read, actually Aelle is not really a bad guy in that book, and Derfel, the hero is half Saxon. The baddies in that series are actually the Christians (as so often in Cornwell's ouevre).
He actually missed a trick, Cerdic, the bad, rat-faced king of the saxons was historically almost certainly a Briton who led a mixed Saxon/British force. Having him as Romano-British traitor would have been a cool addition to the book.
Cerdic's name was British (Ceredig), as were many of the names of his early descendents. Richard Coates argues that his people were the Gewisse, meaning "the known ones", and that they were a group of Britons allied to the Saxons, based around the old Roman city of Dorchester on Thames. If that is true, they almost certainly converted to Paganism from Christianity.
Also, Penda of Mercia's name means "Good chief" in Welsh, and his father's name was Pybba, neither name has a Saxon etymology.
It makes you wonder what exactly was going on in 6th and 7th century Britain.