Yeah the police in airports/around London (CO19?) use G36s and MP5s. Don't know much about the different versions, I just know that the assault rifle G36 is a G36E and I think that's what airport police use (it's got an intergrated scope) and the MP5 has a retractable stock. Anyway, yeah the SAS use whatever they like but it is usually the Diemaco C8 and MP5, they did use Minimis and a GPMG (general purpose machine gun. aka GIMPY I think, lol gimp)in the Bravo Two Zero mission although Minimis were replaced...or was that the GIMPS? I think both have probably been replaced now.
I know most of us airsofters would like to look realistic but I don't think there's much chance of getting SAS stuff, ex-Army (GI) stuff yeah there's loads of it, but real SAS stuff must be hard to come by, and expensive.