Also, a further note, the Anglo-Saxons only became Christian a bit further on in the 7th and 8th century, quite a bit after their initial conquest of the Britons in the 5th and 6th centuries.

The Britons (later, "Welsh") themselves were already primarily Christian, due to Roman occupation. They saw the Saxon invasion as a catastrophe not just because of the cultural displacement, but because it was pagan. (cf. Gildas "De Excidio et Conquestu Britanniae")