British sound???

Speakers that emphasize the midrange. Often monitors and lower wattage integrated amps. PRaT as mentioned above is a trait.

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.