[...] Christianity didn't make them into the people they were, they were already those people. Germans were Germans, Brits were Brits, their languages may have ended up different, but we allow for that in a role playing game.
Again, you seem woefully misinformed on fundamental historical points. The English are predominatly defined - culturally if not entirely genetically - from Germanic tribes that invaded Britain. The Brits are Germans - within your Roman definitions.
https://en.wikipedia.org/wiki/Anglo-Saxon_settlement_of_Britain
The Anglo-Saxon evidence is overlaid by effects of the Norman Conquest - but the Normans were NorthMen who had first moved down to help shape modern Fance. Thus again people from outside the Roman frontiers.