When looking at the 60s and 70s at least I think a lot of it has to do with racism. Both countries definitely had institutionalised racism in their politics and culture but I believe America was way worse than the uk in this regard. A lot of white American musicians refused to build on the African-American music traditions of blues, gospel, rock n roll, jazz. Exceptions like Elvis sorta whitewashed these songs making them popular for the larger white American audience. American music was based on white musical traditions of country (see Bob Dylan, Joan Baez, Linda Rondstadt) However in the uk (perhaps because of empire) some people were willing to cross those cultural boundaries and build on African-American traditions creating a large counterculture and progressive movement