The early Angles, Jutes, and Saxon settlers were pagan themselves, belonging to the long tradition of Germanic paganism. Christianity would not really begin to replace Anglo Saxon paganism for several generations, with St. Augustine being the to make significant headway in converting.

Even after the conversion of the Aethelberht, it took many more generations for all vestiges of paganism to disappear in England. As such I am not sure we could fairly call them "settlers" by that point. We are talking well into the late 7th C at the least, (King Arwald for example).

Native Briton paganism had been under pressure from before the arrival of the Angles, Jutes, and Saxons. Irish monks had been bringing their brand of insular monasticism. As such, if anything I'd argue that the Germanic settlers represented a reinvigoration of paganism on the island.

edit: (I'm finishing up my BA in History, and its been a few semesters since I've covered this material so forgive if its a bit brief... It's early morning to lol)

Rome was technically Christian, so this is the default religion when the AS come in, although there's a good case to be made that it's not clear how far Christianity penetrated as the Romans never fully subjugated Britain. We've got British bishops as early as 314AD, a large number of monks and monastic orders from about the 4th century onwards, but the AS invasion was a pagan invasion itself.

Your question assumes that the native Britons were pagans, which I'm not sure is the case. Can someone tell if my assumption, that the South of Britain of was the more Romanised and also Christianised part, is correct?

Cornwall quickly evolved into a Christian kingdom during the early post-Roman period and it had been one of the least romanised parts of Britain -- there was no Roman town West of Exeter. At Tintagel, they traded with their co-religionists in Byzantine North Africa into the 6th Century and Byzantine wine amphorae for instance are found at high-status sites throughout Ireland and (especially) in Western Britain. (I found this but I don't know how recent it is.)

If the question is whether Roman penetration determined the degree of Christianisation, this example suggests it did not. On the other hand, St Patrick was a highly-Romanised Welsh man with good literary Latin and AFAIK is considered typical of the church hierarchy.