I don't know about today, but being an enthusiast of older music I know that the U.S. often tended to have regional hits back in the 1950s/60s, due in part to the numerous small, local record labels. Sometimes a singer or group would get a lot of radio airplay on the West Coast but not in the East, or vice versa, or a song would only become a big nationwide hit if one of the larger companies (Decca, RCA, ABC-Paramount, etc.) picked up on the local popularity and starting selling copies widely across the country.
That's in slight contrast to -- to use Vicky's example -- the Merseybeat genre of the sixties in Britain, where suddenly groups from that area were all the rage but the records sold right across the country pretty much from the outset. There were relatively few small, independent labels such as those which could be found by the hundred in America at that time, so once Parlophone, H.M.V., Decca, or whoever picked up the song here, it would soon be sold right across the country.
I think another consideration for that time was also how people got to hear the music. Choices in Britain during that Merseybeat era were extremely limited, outside of live music or the ubiquitous jukebox. The BBC and ITA had a couple of weekly pop music shows on television (Six-Five Special, Juke Box Jury etc.). Radio had nothing like the choice of the U.S. at that time, with just three official BBC stations, only one of which broadcast "pop," and even that was severely limited in its content.
There was Radio Luxembourg, and then the pirate stations caught on in a big way from about 1964 (Radio Caroline & Radio London being two of the most well known), but that still doesn't give a huge range of stations, and still not on the regional American basis.
BBC Radio 1 started in 1967, but on a national basis, and even when BBC and ILR stations opened during the 1970s many were little more than regional variations of a cross between Radio 1 and Radio 2.
The explosion in the assortment of "genre stations" which can be found around the U.K. today didn't happen until the 1980s when licensing conditions were drastically relaxed.
I would say that this history of British radio broadcasting has probably played a fairly large part in there being less regional variation in the U.K. (as well as the compact size of the country, of course).
