Well nobody knows just how many Britons popped over to Brittany to escape the Anglo-Saxons. Might have just been a few important ones(with bigger swords than the natives)...enough to import their language, but not enough to make much genetic impact. then again, one theory is that the Welsh actually moved down from scotland into Wales(squeezed by the arrival of the Anglo-Saxons from the south andthe Scots from Ireland who all arrived simulatneously in a rather anti-social fashion. I believe there is a lot of literary eveidence to support this view.
    Gentics in Wales is bound to be a bit confusing anyway, due to the practise of interbreeding with animals.