I don’t think immunity from war crimes can be guaranteed to anybody.
The SAS is nobody’s fool—nobody sane wants to go to the Eastern front.
It’s not about outright war crimes, these SAS troopers ambushed a group of IRA operatives coming back from attacking an RUC station with heavy machine gun and AK74s and was ambushed by the SAS on the way back.
“was not planned and controlled in such a way as to minimise to the greatest extent possible the need for recourse to lethal force”.
Can you imagine rules like this being applied in a war zone? The IRA fancied themselves as soldiers fighting on the battlefield. In a war you destroy the enemy when the opportunity presents itself, you don’t mess about trying to arrest them when they are an armed threat.