The Special Air Service currently uses the Canadian Diemaco C8 SFW as their primary weapon. They also use the Belgian designed and British built FN MAG, the Belgian FN Minimi, the Swiss SIG P226, the British Arctic Warfare rifles, the German AG36 grenade launcher and so on. The German H&K HK53 may well still be in use for personal protection routines. In fact, I can't think of a single American hand-held small-arm used by Them. Of course, this is all based on what we're told by the MOD, ex-SAS members and what we see in the rare glimpses of Them caught on film. They could be using American 180s and we might not know about it.
The SAS were named the SAS because that's what they did - special airborne operations.
As a rule, anybody who thinks they know anything about Them beyond what they see on TV, read on books and on the internet is talking rubbish. Ever pub in Britain has a bloke who used to be in the SAS or whose brother was in the SAS but got mortally wounded by an Iraqi in Desert Storm and told him everything he knew on his death bed, or who is technically still in the SAS because he jumped out of a moving helicopter to escape the genetic experimentation the MOD was conducting on Them, or who sat and shat the **** with them in the Falklands for a couple of hours and thought they were "alright fellas but a bit detached if you know what I mean" and they let him hold their magic ray guns while they took a wee and they asked if he wanted to join but he said he preferred the quiet life.