What was so different about Roman and Post Roman Britain?

From modern population studies the English average 38% North Sea Germanic ancestry. The most recent study also points to a substantial post-Roman influx of Iron Age Gaulish-like ancestry, probably via Frankish mediation, not recognised before. 

Ancient DNA studies have shown some Anglo-Saxon settlements to have been composed of 100% Germanic incomers, while others show that native Britons became culturally assimilated to Anglo-Saxon norms and produced children who had hybrid ancestry at a very early date. Men of native ancestry were buried with weapons in the same way as men of Germanic ancestry, there seems to have been little discrimination in these communities based on ancestry.

The Brythonic Celtic names found in the early members of the West Saxon royal family - Cerdic, Ceawlin, Cedda, Caedwalla - show that in some areas native families retained political power and slowly became anglicised. The names of the Mercian kings Pybba, Penda and Peada look far more Celtic than Germanic, perhaps indicating some native British connection for that royal dynasty also. It is notable that the warlike Penda never fought against the British/Welsh, but was always allied to them in wars against the Northumbrians.

This period is, as you may have guessed, one of my 'special interests'.

From modern population studies the English average 38% North Sea Germanic ancestry. The most recent study also points to a substantial post-Roman influx of Iron Age Gaulish-like ancestry, probably via Frankish mediation, not recognised before. 

Ancient DNA studies have shown some Anglo-Saxon settlements to have been composed of 100% Germanic incomers, while others show that native Britons became culturally assimilated to Anglo-Saxon norms and produced children who had hybrid ancestry at a very early date. Men of native ancestry were buried with weapons in the same way as men of Germanic ancestry, there seems to have been little discrimination in these communities based on ancestry.

The Brythonic Celtic names found in the early members of the West Saxon royal family - Cerdic, Ceawlin, Cedda, Caedwalla - show that in some areas native families retained political power and slowly became anglicised. The names of the Mercian kings Pybba, Penda and Peada look far more Celtic than Germanic, perhaps indicating some native British connection for that royal dynasty also. It is notable that the warlike Penda never fought against the British/Welsh, but was always allied to them in wars against the Northumbrians.

This period is, as you may have guessed, one of my 'special interests'.

I've always wondered about the Brythonic names of West Saxon leaders too. I wonder if the Saxons these people were leaders of were foederati, mercenaries paid with land grants and cash in the Roman fashion, it would certainly make sense of the old tales of Kings like Vortigern?

I think it was assimilattion rather than conquest during the earlier period, obviously later it did become violent as Anglo-Saxons took control, we have fairly firm dating evidence for the fall of Romano British towns like Cirencester and Bath.

As for language I wonder how many languages were spoken in Britain at the time? Did we adopt Saxon languages in a sort of rebellion against Rome and Latin? Did the incoming Pagan A/S refuse to use Latin because of it being a religious language?

I think that as we don't really have a specific A/S gene marker it's hard to tell who was who, if we can get Stable Isotopes then we can tell where people grew up, but that dosen't tell us how they came here and thier children would have the stable isotopes of the area they grew up in. A lot of the archaeolgical evidence can be confusing too, is a grave with no grave goods a sign of a Christian or someone at the bottom of society with no possessions, such as a slave? Even were we do have grave goods it can be a confusing picture, were "Germanic" goods fahion, a sign of ancestry, a sign of mixed marriage even? TO give an example, would a future archaeologist wonder if we had been conquered by China and an Empire in South East Asia because so many of our househoid objects come from that area, if there were no documents, how would they know? The picture changes as you move westwards too and in many ways gets more confusing, I've thought for a long time that we should stop thinking about a north/south divide in favour of an east/west one, certainly for pre-industrial revolution stuff.

One of the other things I wonder about is religion at the time, we're told that the Romano-British were Christian, but were they, and what sort of Christianity was it? Also why does there seem to have been no efforts by the Romano-Britons to convert the Anglo-Saxons?  Was thier Christianity more akin to that of the Irish Church with it's differnt dating of easter, different tonsures and abbots instead of bishops, or was it more akin to that of later missionaries such as Augustine?

I would also ask, how widespread Christianity really was in Britain during late antiquity? We know that the empire was officially Christian but how was it in practice? Did other religions still survive in their previous form or did they go underground and become as secretive as Chritians had had to be previously, or was it hybrid?

It's thought by some that Hengist and Horsa the guys who were said to have led the invasion of Britain, were actually the name's of now forgotten gods.

The other thing is that when people think of Britain they tend to think of modern Britain and it's political boundaries, it's shires etc, but that wasn't the case then, there were areas ruled by all sorts of different people, from war lords to petty kings and the remains of Roman civic administration.

Britain deromanised faster and more completely than any other province and given the number of legions based here, was it ever fully Romanised? My feeling is that it wasn't, hence why we see a return to pre-Roman territiorial boundaries, the reoccupation of hillforts and it would seem any real will or wish to go back to being part of the empire?