
One would measure the indigenuity/indigenousness/indignancy (fuck knows, can't be bothered to check) of the a population via their DNA - not their habits, customs, and language.
Oddly enough, studies have in fact been done to this end - and found that successive waves of immigrants (Romans, Saxons, Vikings, Normans, and so on) have made far less of a dent in the original 'British' gene-pool than anyone had expected.
I'm not making any sort of political point, it just struck me as interesting...