Oh, the degree of inter-marriage across cultural groups in Europe is huge, much larger than most people realise. And there is a 'romance' about being a Celt right now, so people are attracted to it. I am probably mostly a Celt -- Welsh mother, father from the Herefordshire borders -- but there's Somerset in there too, and Black Country, and on both sides we have blue eyes as a recessive (I have them, along with 3 cousins). IN the early middle ages, these areas were probably fairly mixed anyway -- Britons and Saxons -- and then we get the Scandinavians and the Normans (Franko-Scandinavian with some Breton) and the Flemings and....
We're a mongrel race, the British.