[Bayart]

Nobody can. There's far too much overlap regionally between Britain and North-Western France.

And nobody in Britain has just Brittonic ancestry, or Frankish ancestry in France. For the most part the populations in Europe have been stable since a time that predates the expansions of the Celtic and Germanic linguistic groups.

[laurencerowe]

Studies have found genetic evidence for population movements. You could make an argument for the Welsh having overwhelmingly Brittonic ancestry.

https://peopleofthebritishisles.web.ox.ac.uk/population-gene...