A recent genetic study published in the journal Nature has found that before the arrival of the Norse and the Saxons, the Briton or ‘Celt’ populations in the British Isles did not belong to a single, homogenous ethnic group. Researchers have found slight genetic variances in non-Saxon populations which indicate that the Britons were not one people, and instead were comprised of many sub-groups. This find reinforces a proposed theory that Britons self-identified as ‘Celts’ more in the national sense rather than as an ethnicity; not unlike self-identifying as French or German today, regardless of ethnicity.
While some have questioned the finds of the researchers, the genetic tests confirmed previously well understood genetic origins of the U.K. Among the finds, over one quarter of the population derives some genes from the conquests of the Vikings, making them one of the largest ethnic groups to have lent their genetic makeup to modern Brits. This means that if you are from England or Scotland (not so much Wales), you very likely have Viking blood in you.
In a period referred to as the Danelaw, Danish conquerors controlled vast swaths of Britain and settled in large numbers in those areas. This period is attributed as the major cause of Scandinavian genetic markers in modern Brits, though the later Norman invasion of 1066, which established Norman law, also contributed.