Reading both Bryan Ward-Perkins’ 2000 paper Why did the Anglo-Saxons not become more British? and The fine scale genetic structure of the British population, published in 2015, is interesting. To date, this second paper is probably the “best of breed” when it comes to estimating Anglo-Saxon admixture into the British population in the 5th to 7th centuries (confirmed with a smaller sample ancient DNA publication). The authors conclude that:
Two separate analyses (ancestry profiles and GLOBETROTTER) show clear evidence in modern England of the Saxon migration, but each limits the proportion of Saxon ancestry, clearly excluding the possibility of long-term Saxon replacement. We estimate the proportion of Saxon ancestry in C./S England as very likely to be under 50%, and most likely in the range 10%-40%.
The ancient DNA paper gives an estimate of ~38% Anglo-Saxon (German) for the “East English.” So the two seem roughly in line. The C./S. England cluster refers to the genealogical network of the lowlands of central and eastern England.
There are several ways we can look at this. First, the majority of the ancestors of the modern English were British. That is, Brythonic people of various levels of Romanization. They became Anglo-Saxon. Even on the “Saxon Shore” in the far east of England it is likely that the majority of the ancestors of the natives derive from post-Roman Britons (if barely).
A second way to look at it this that this validates Peter Heather’s model in Empires and Barbarians: The Fall of Rome and the Birth of Europe. The model being that the post-Roman barbarian migrations were coherent “folk wanderings,” and large numbers of Germans moved into the collapsing Roman Empire. In post-Roman Britain, a large number of Germans clearly arrived and demographically marginalized many Britons. To be sure, it is unlikely that in the year 550 AD the census size of Germans to Britons in East Anglia was ever 38 to 62 in ratio. Rather, I suspect that in the centuries after the rise of the Anglo-Saxon kingdoms German elites had higher reproductive rates than the Britons due to their superior access to resources. Over time this resulted in their contribution being more prominent in the genealogies of people alive today.