...I admit some serious over-simplification being well aware of the shift in makeup over the years, both in antiquity by amalgamation of the original peoples, then the invasions from without by the Scandies, Romans, Normans up until (perhaps arbitrarily) 1066, then after the Civil War(s) arguably as a result of colonisation.

Perhaps the blended vs. mono-cultural bit is indeed oversimplified or even overstated in the case of the UK, though one cannot help but have noticed there was a very definitive sense of what it was to be English or Scottish or Welsh anytime I was there; the not so old joke about who will be the first brown man to move into Midsomer and all. Not so in America, unless it is a militantly white Christian claiming mono-culturalism as his own. The rest always refer to themselves as Afro-american, Chinese-american, Irish-american &c.

Britain, having been a colonial power and a seafaring nation made wealthy in commerce (the two very closely associated) isn't as monocultural as, say, Germany as a landsman's country, or Japan as a long-isolated one. There is a matter of degree I ignored simply because that tome was long enough. GRIN!

Still, the point is intended as the Americans began as an experiment in immigrants and a cultural blend, so the social and legal concepts are unique to that purpose and get held in different levels of importance from other cultures, to a degree even Americans won't acknowledge when looking at others.