the SAS/SBS uses the MP5 as does most SF fores of the western world or uses the G11, sometimes they use M4's or M16's but mostly it depends on the situation.
mostly though its MP5's
I like to point out that special forces or any forces in the world for that matter, actually use the G11. The H&K G11 (which I believe you are referring to) was a caseless prototype rifle that never saw production. Possibly one of the greatest guns ever made, but sadly, funding was cut with the fall of the Berlin Wall.
As said before, the SA80/L85 has a lot of problems, enough that units with more flexibility in their arsenal tend to look towards alternatives such as the armalite series.
Bullpups have the advantage in a longer barrel in a smaller package, making the gun more manueverable with the smaller package and more accurate with the longer barrel. They do have some problems though, one being it is hard to switch shooting hands in the heat of the battle. Guns like the Steyr Aug can eject either left or right, but require a cool to do so, which isn't possible when you are getting shot at. The long term effects of shooting with the mag right next to your ear are still unknown.
So.. tradeoffs.
Yea, i'm new here, hello.