I’m wondering how many truly British people there are. Is it an irreversibly dying ethnic group?
90% are truly British.
Remember that even in 1900 the British people were mixed.
It is not a dying group
Ethnic groups change.
What I find fascinating is that in 1077 you had in England a mix of angles, saxons, Danes, Welsh and ruled by French Norman’s. In Scotland you had the scotti, oicts, Britins from Aukd clutch what is now Glasgow area, with normans in the Edinburgh area and a bunch of Danes too.
Both these nations got created by the 14th century.