British sound???

some say they emphasise soundstage/imaging less in addition to PRaT

British sound is a relatively neutral signature with good PRAT and musicality. They dont sound like typical European products which try to be very sophisticated, Brits try to be honest.

The BBC established specific design criteria for several monitoring situations. There's a whole series of them - 3/5, 3/6, 5/1, 5/8, 5/9, etc. I believe the numbers have something to do with the intended size of the studio where they will be used? It's a midrange neutral design as opposed to the midrange forward design favored in the USA by Western Electric/Altec/JBL.

Yes, and hearing too. Many years ago FI magazine ran an article about how ones hearing is influenced by the county you came from. So, Brits do hear differently and from that 'British' oriented speakers involved. One of the best illustrations of that is ProAC speakers.

I wouldn't say that they emphasize the midrange, they're main concern seems to be getting the midrange right. Doing this, may be at the cost of less treble or bass, since that seems to be their primary concern. With that said, the bass or treble is not wrong, just not equally emphasized as American, or some others countries speakers may be. The bass and treble might sound lacking a little when compared to a lot of non-British types. They can be very accurate since the most important part of the music is in the mid-range area IMO. In the past years, they've been doing a better job of getting more the whole range included, without sacrificing their accurate mid-range. Spendor and Harbeth seem to go more in this direction, and less so with B&W, or some other brands.