In early days many Britons were enslaved by invading Saxon forces, although slavery in Ancient/Early Middle Age times was in many ways different then what we usually expect based of what we know of slavery in US. It wasn't really based on race/ethnicity/religion as much as being out of luck and owing money, or being simply on the wrong side of the war. Slaves were often later (if they survive long enough) accepted into the society and over time they would become fully assimilated into the society - some would even rise to become very respected people.

You can find some interesting facts on this subject here: https://octavia.net/slavery-in-anglo-saxon-england/

Mass slavery of the Roman form wasn't "popular" among the West Germanic tribes because they had their own system of subjugation of a more feudal model, or at least something transitional between the classic Germanic class system (king, nobility karl/jarl, freemen, laymen) and what became feudalism.

Or rather, after their brutal occupation and displacement of the Romano-Britons they came to emulate the Franks (the most successful post-Roman polity in Europe) in the structure of their economy. Think Serfs rather than Slaves.

Slaves in the Roman system could be freed and become Roman citizens. In fact it was rather common after a generation or two. Peasants were peasants forever. Not saying one is more just than the other, but Christian influence in Europe certainly hadn't made for a more just system. Just a more theologically based one (God chose your King and Lord)

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")