TL;DR maybe.

If you found the short answer to be wholly unsatisfying and nearly useless, you can join the rest of scholars that feel the same way, because that time frame was called the dark ages for a reason. There are VERY few sources at this time and many of them are contradictory, incomplete or both. We really don't know the extent of "romanization" of the British, or even many details of their Celtic religion. Our dearth of written sources is somewhat supplemented by archaeological evidence, but that is harder to read than what few words we have from the era. A good book on the subject is Britain after Rome by Robin Freeman, it largely focuses on Anglo-Saxon Britain, but the first two chapters deal with Roman Britain and then their leaving in some depth.