It’s mostly cos of incest.
I think the Germanic tribes that conquered the natives made incest prevalent cos they weren’t that many & they didn’t intermarry with the natives (they kept marrying one another in incestuous relationships). And that permeated the whole society.
That said, there are tons of very attractive British cacs (women), especially the middle class ones. I saw a lot of them in Uni. My first graduate job was also in the city (that’s like Manhattan in the US) & there were so many eye candy British cacs.
You won’t really see attractive ones amongst British working class cacs - you’d have to go to middle class areas outside London or farm areas deep in the country.