it goes back farther than that even
"my great grandfather was British"
OK was he English, Scottish, Irish, Welsh, possibly even Manx? was he a mix of those? I bet he was
let's say he's English, what does that mean? England has been invaded tons of times by different groups that affected its genetics
oh you say your great grandmother was French? what kind, was she from Normandy, Provence, Alsace-Lorraine, Brittany, maybe even Basque country? was she a mix? I'm sure she was a mix, and what does that even mean? her Alsace-Lorraine part was surely part German, her Provence part may have been part Italian, her Norman part was Celtic, and we don't even know what the fuck the Basque are
you can do this for any country
sometimes they're isolated like Japan, but even then there's shit like Hokkaido and Okinawa that are considered to be Japanese in every practical sense