Noah Carl writes:

In a paper published in Nature, Leslie et al. (2015) report that there is very little genetic structure (differentiation) within the native British population:

Consistent with earlier studies of the UK, population structure within the PoBI [People of the British Isles] collection is very limited. The average of the pairwise Fst estimates between each of the 30 sample collection districts is 0.0007, with a maximum of 0.003.

This means that native Britons living in one particular area of the country (e.g., Orkney) are not much more closely related to their immediate neighbours than to Britons living in a completely different area of the country (e.g., North Wales). In fact, the largest pairwise Fst values (roughly the proportion of variance between groups) were for Orkney versus North Wales (Fst = 0.003), and for Orkney versus North Pembrokeshire (Fst = 0.003). By comparison, Fst values for major continental groups (i.e., Europe, Africa, East Asia etc.) are in the range of 5–15% (>100 times greater than the average for areas of Britain).

The findings adduced above make sense when one considers the magnitudes of historical migratory flows into Britain. Estimates for the fraction of the population that Normans comprised, following the Norman conquest in 1066, range from 1% to around 5%. Between 1066 and the turn of the 20th century, it is unlikely that the foreign-born fraction of the population ever exceeded 2%. French Huguenots, for example, are unlikely to have constituted more than 1% of the population. …

The magnitude of inward migratory flows increased during the 20th century, and did so dramatically from the 1990s onwards. Between 1900 and 1950, the foreign-born fraction fraction of the population rose, but never exceeded 5%. By the early 1990s, it was well above 5%. In 2011, it was around 13%. And today, it is probably above 15%. Thus, contemporary levels of immigration into Britain are historically unprecedented.