I don't want to get into the whole DNA thing - it's complicated, and potentially controversial (although interesting). But I've recently been reading "The Age of Arthur: A History of the British Isles from 350 to 650", by John Morris. My copy has a copyright date of 1973, but it's in the name of his wife, so I wonder if it was originally written somewhat earlier.
It's an interesting, altohugh somewhat difficult book to read, partly because of its length (665 pages including index and appendices), but mostly because of its structure. It jumps about a bit, and also assumes some previous knowledge that I don't have.
Anyway, he seems firmly of the opinion that there was no mass "ethnic cleansing" of native Britons by the so-called "Anglo-Saxons", and I must say, this supports my own prejudice somewhat that the native Britons didn't all flee from what became England, but quietly adopted the language and probably the cultural ways of the invaders.
However, there definitely was some emigration to Brittany (co-incidentally, we've just spent some days in north-east Brittany, so this was of more interest even that it might have been). He talks about there being three distinct waves of emigration, and it seems to have been the last one that had the most impact, in making that part of Gaul/France "little Britain beyond the sea".
Surprising to me was the suggestion (if I am reading him correctly) most of the immigration might have come from Wales, rather than Cornwall, as I had supposed (especially since Breton is said to be closer to Cornish than Welsh).
Well, Wikipedia claims that most of them came from Cornwall and Devon. I'm not going to argue (but St Malo came from Wales - https://en.wikipedia.org/wiki/Malo_(saint) ).
I must re-read parts of that book to get a clearer idea of what he says about the immigration.
From Wikipedia: https://en.wikipedia.org/wiki/Brittany#Religion
Bretons are mainly Catholic and the Christianisation occurred during the Roman Gaul and Frank era. During the Briton emigration to Brittany, several Christian missionaries, mostly Welsh, came in the region and founded dioceses. They are known as the "Seven founder saints":
(Of the 7 listed, 6 seem to have come from Wales).