British ethnicity is complicated. English, Scottish, Welsh, Irish? If Northern Irish then loyalist or republican? Do you count the Cornish separately? Germanic or Celtic? Norman ancestry.

Its not defined by genetics. A very high proportion of "white British" people I know (that is of those who have told me!) turn out to have all kinds of ancestry (Indian, Romany, and Polish examples come to mind) in recent generations.

Genetics do not refine race/ethnicity, culture and cultural definitions do. Americans have definitions even the culturally similar British do not understand (e.g. hispanic, white passing black). I am defined differently in different countries: https://pietersz.co.uk/2023/08/racism-culture-different