I think this honestly applies for every country though, however it's impossible to recognize unless you grew up in said country, or lived there for many, many years. I would say the UK easily gives the US a run for it's money - but again that's only because I've lived my whole life there and have learnt to recognize the differences. The cultural disparity between Newcastle, Edinburgh, London, Cornwall, Liverpool, Belfast, Leicester, Birmingham, Inverness, Manchester, and all the small towns and villages in between are so vast, despite being so close geographically; the UK is only the size of Michigan, yet travel 40 miles in any direction and the dialect, food, buildings, clothing, and music preference can be completely different. The trip from Chester to Caernarfon is only as far as travelling half way down Long Island, but in one place they speak almost 100% English, and in the other it's more like 20.
Granted we don't have the dramatic shifts in climate (although our microclimates are quite unique), but the population density and long, chequered history as a nation seem to make up for it. Any country, despite how unified they look on the exterior, is not a realistic representation of the country as a whole. I'm sure a person from Moscow has very little similarity to one from Vladivostok, or likewise Sao Paulo to Sao Lius. There's no where in the world you can visit, besides some microstates, that give a realistic representation of the country in one visit.
American who lived in the English Midlands for a half year.
Your hunch is correct: Britain absolutely has more cultural variation than America. You see it in the accents the most. I never expected to have difficulty communicating with other native English speakers, but I was comforted by the fact that other Brits from different parts of the island struggled to communicate with each other a bit as well.
Granted we don't have the dramatic shifts in climate (although our microclimates are quite unique), but the population density and long, chequered history as a nation seem to make up for it. Any country, despite how unified they look on the exterior, is not a realistic representation of the country as a whole. I'm sure a person from Moscow has very little similarity to one from Vladivostok, or likewise Sao Paulo to Sao Lius. There's no where in the world you can visit, besides some microstates, that give a realistic representation of the country in one visit.