The movements of people since time immemorial does not adhere to the arbitrary political lines we've drawn between nations today. Both France and England have seen large scale immigration by the Romans, various old Germanic people, then the Vikings. All these people have killed and fucked each other. Attributing DNA to an area is partially a statistical likelihood, so there's a margin for error. Except in geographically and/or historically isolated areas, we're all more blended than anything else. That makes the race theory of the late 19th century seem so utterly ludicrous today but we can't quite completely get it out of our heads either.