the Main British Infantry Sidearm is the Browning 9mm.
If you're going for an SF look, then it's the MP5A4 or a 203 (an m16 with a full 203 front end)that you'll be using - they're better than the L85 any day.
The SAS are normally issued with Sig Sauer P220 if they want something with a heavy punch, and aren't so bothered about concealment.
Or they'll be issued with the Sig Sauer P226 - A lesser Calibre means less of a punch, but much more effective in an urban environment.
As for the Glock 17 - these are only issued to Soldiers/Operatives who are working under A Section 5 license in the UK, as MI5 can therefore cover them under their deniability clauses.
Sources for all this information:
A friend.