Every time i see the show they are using the American M4. I realize that this is sometimes necessary because the SA80A2 doesn't fit a grenade launcher, but really can't they use a weapon made in the UK?
*Forever Cho Chang Fan*
Cho's story continues at: http://www.fanfiction.net/s/1552296/1/
Actually they can:
http://www.historyofwar.org/Pictures/pictures_SA80A2_UGL.html
As to why the SAS dont use it, well there are a myriad of reasons, ranging from the (poor) reliability of the weapon when it first came out which gave it a bad repuation
the loud noise the safety catch makes which could prove deadly given their operations
the heavier weight which is very important if you are already carrying mountains of kit as they tend to do
and of course, the importance of being different to the regular soldiers.
The AR15 family of rilfes had been around for a long time, and was tried and tested, the SAS knew that they were doing and were comfortable with those weapons. Changing for the sake of it doesnt make any sense which is why they have always been permitted to select their own weapons.
They do use some British weapons, but not many, primarily sniper rifles produced by Accuracy International.