National Servicemen in the SAS
One gets the impression from the movie that all the members of the SAS were regular soldiers, ie volunteers. However according to David Horner's book "In Action with the SAS" by 14 April 1971 25 out of 95 in the squadron in Vietnam were draftees.
I imagine they would have volunteered internally for the SAS? It hardly seems appropriate to send unwilling conscripts on small 10 day patrols deep into the jungle among the Viet Cong.
The casualty rate in the movie seems rather high also. There seem to have been about 35 serious casualties in the 100 man unit over about 5 years, which would be of the order of 5-10 percent per year. In the movie, the troop has about 80 percent casualties in one tour, mostly killed.
The selection criteria for the SAS would appear to emphasize great determination above all else.This seems at odds with the movie's anti-war message. Perhaps an infantry unit would have been better for the setting?