There's certainly evidence that (part of) the literary tradition came from the North Britons, but none of mass migration. Which there is in Brittany. If the genetic traces point to a basque-ish origin, this does imply a long-lasting population. The Beaker people? This reminds me of the recent DNA match between prehistoric man in Somerset and a current inhabitant. (Though none of the people I knew in Bristol would have been in the least surprised at that.)

It has long been argued whether the Celts were a population movement or a cultural one: it seems that in Wales it was mainly cultural, whatever it was in the rest of the country. The Cornish and Breton DNA evidence might add more to this - shame no-one seems to have seen this.