Perhaps people more familiar with the British education system elaborate but you used to hear there were more “art and music” schools in Great Britain, at least in the late 1950s/early 1960s. A number of groups in that era met in schools and formed bands while in their late teens.
I mean that 75% to 80% of the music scene (as measured by the percentage of musicians, of music sold, or of music listened to at public events) in the U.K. could reasonably be considered rock and roll. In the U.S., it’s something like 25% to 50%. It depends on what you consider rock and roll to be. Rap is an off-shoot of rock and roll, and at first it was considered part of it. Some people don’t consider it part of rock and roll anymore. njtt doesn’t even consider whatever Whitney Houston and Mariah Carey sing to be rock and roll. And certainly country isn’t part of rock and roll, since it slightly predates it.
There may be a lot of diversity within rock and roll in the U.K. There might well be more diversity there than within rock and roll in the U.S. But the point is outside of rock and roll, there is a lot of diversity in the U.S., mostly in country and rap, and there’s not nearly as large a proportion of that in the U.K.
“Disproportionate”? The UK looks small on the map – Florida alone is larger than England – but its population is 62 million (almost twice California’s). Compare that to (only) 313 million for the whole U.S. and you’ll see how crowded is Britain.
What does crowdedness have to do with diversity? Once again, what I meant by diversity is that there are genres usually considered outside of rock and roll that are huge in the U.S. (in particular, country and rap) which are minor in the U.K. Comparing 62 million to 313 million isn’t the correct comparison. What you should compare are 62 million times 75% to 80% with 313 million times 25% to 50%, since the proportion of the U.S. that considers rock and roll to be their favorite music is only 25% to 50%, while the proportion the proportion of the U.K. that considers rock and roll to be their favorite music is 75% to 80%.
Once again, if somebody is going to say, “Country and rap can’t really be that big in the U.S. Nobody is starting threads about them on the SDMB,” then you’ve got to understand that the SDMB isn’t remotely representative of the American population. Country and rap are huge in the U.S. The sort of American who posts to the SDMB doesn’t like to admit that. They want to think that the people that they live near are representative of the entire U.S. They aren’t actually. There are many neighborhoods where people listen mostly to country. There are many neighborhoods where people listen mostly to rap.
I can’t compare the two but I can certainly offer a view on why the UK music industry thrives so much. While you’ve raised a valid point that instrumental teachers are just as good in America as they are in the UK, I think it’s an irrelevant point. I can’t speak for the American system but in the UK young people are given music education in a much more thorough way than just tuition with a teacher.
Literally just this weekend I was playing at a local music festival in an hour and a half slot that my school had been given
The groups playing were different combinations of kids aged 12 - 18 well as a few alumni of the school that had come back from university/military-college specifically for the festival slot/rehearsals. You’d be surprised at how effective this is at motivating kids to become serious about their instrument (or voice) - all of the music comes from collaboration between the students with older ones (myself included) sharing as much knowledge as they can. Two bands that formed at this festival slot 5 years ago have since played at the Glastonbury Festival acoustic tents several times and get plenty of other gigs elsewhere. But, this is only one example of many opportunities young musicians get in the UK - another is that despite my school supposedly being broke, the music dept recently a whole suite of Macs with fully licenced professional music software and recording equipment (in retrospect this is perhaps why my school now has no money 
).
Another benefit that young UK musicians receive I’d the huge market for underground artists. Anyone who’s considered “cool” at my school listens to bands that most people wouldn’t have heard of and spend a lot of their money on concerts to see these bands.
Anyways sorry for the long response but feel free to comment on the American system so there’s an actual comparison 
.
I only skimmed through the answers so far, so this may already be brought up, but my working theory (okay, WAG), is that most good rock and popular music comes from somewhat impoverished, hard-working, blue collar environments. So it’s not so much that Great Britain produces great rock/pop musicians, but the hardscrabble, working towns in GB produce them. The Beatles came from Liverpool. I would guess that many of the great British bands came from similar upbringings. Yes Mick Jagger and I think Keith came from upper-middle class backgrounds, but I’m guessing they’re the exception. On this side of the pond and my area of the world, a lot of great bands / musicians have come from the Detroit area.
The reasons would be: young people are trying to escape their fate as working drones, and I think their upbringing provides more creative fodder and inspiration for good music. On the other side of the coin, I think it’s part of the reason why successful acts usually have a creative dropoff later in their careers- “ivory tower” syndrome sets in. They’re no longer living lives that normal people can identify with.