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.

Can they? They clearly want you to think so, but my own personal results are pretty mixed on how accurate that is.

And yet the genetic differences are so insignificant so as to make them pointless to mention in the context of (paraphrasing) "it's super strange that German immigrants speak the Germanic English language that originates from the cultural region of Northern Germany".