i think the "brit sound" characterization comes from the way the old brit speaker companies', like b&w and kef (and probably some others; sorry if i left someone out), speakers sound. basically, imo, this brit sound lacks the high-end "harshness" and/or "fatiguing" that many people complain about with some other speaker brands. at times this works wonderfully to provide a "mellow", "laid-back", and even more believable sound. at other times, though, depending upon the music, it can result in a perceived lack of bite, crispness, sizzle, or edge in the high-end that those "harsh" speakers do have, and that many are used to.
having recently gone from polks to kefs, i can testify to the distinctly different sound these 2 brands "make". i actually had the polks out and hooked-up the other day, and they do have a high-end bite/shrillness (depending upon how you look at it)that my KEF's don't have.
where these sound differences originally came from and whether these sound differences in these different countries' speakers is rooted in, related to, or influenced by that particular countries' culture is an interesting question.
Most europeans live in smaller quarters than their american counterparts. Reason why they rely almost exclusively with book shelf speakers. Because of their smaller quarters, the preference for more intimate sound. They rather use fabric tweeters than metal.
Most larger market British speakers I've seen/heard use metal tweeters are almost all on the bright side. B&W, Monitor Audio, Mordaunt Short, KEF, and I'm sure others.
The Brits have been making speakers for a long time, and many of the speakers I've heard that I felt were great sounding are British. About a year ago, I heard a relatively unknown Brit maker, Indigo - if you get a chance to listen to them do so. Smooth, excellent detail and imaging.
I went to a guy's house to audition a $30K 2ch system, and he was using some Brit floorstanders that I've never heard of (looked sort of like Thiel, but they weren't). Single 6-1/2" driver and a soft dome tweeter. Quite impressive sounding.
To me, the "Brit" sound is an emphasis on clarity and detial, and posesses a flat overall sound, meaning neither highs or lows are overly accentuated. I find it very pleasant. Of course, not ALL British speakers follow that pattern.
Re: the pop psychology analysis of the British/Australian/Japanese sound... to quote the great Hugh Laurie: What absolute twaddle!
As other posters mentioned, the difference has everything to do with differences in average room size, construction materials, and listening difference. There is also the famous 'BBC dip' in the midrange that I believe most older British speakers had, not sure how common it is now. I found this explanation of the BBC dip, not sure how accurate it is:
"However, if you like the “BBC dip” that was somewhat a trademark of the British sound, it’s nothing more than a 2-3 db dip in the frequency response from about 2K-4K."