the SAS use the Heckler & Koch G3 when they need the heavier hitting power of the 7.62x51, and it would be used whenever its range and stopping power outweigh the weight penalty.
The only time the regular SAS carry the SA80 is when disguised as regular troops although the Territorial army SAS (similar to the National Guard for our American allies/friends

...The Territorials do carry the SA80 as a standard issue. Their biggest complaint is the loud click the safety makes, even on the newer version. 450 pounds ($720) per rifle for the upgrades and its still not right....
Well the click AND the jamming....lol
Poccur