The SAS used to load their BHP mags with 12 rds instead of 13, believing that this made them work better. Maybe this idea got institutionalized. Or maybe they bought spare mags from ProMag - the MoD has been cash strapped lately.

Sig mags work fine fully loaded and can stay loaded just about forever.

The SAS seem to do some odd things by American standards in that video, such as using the trigger finger of the support hand to curl around the trigger guard. They also didn't use the sights of their MP5Ks, nor any sort of optic or laser.

Thanks to a lack of a gun culture, they're probably about 20 years behind the times. Of course the handlebar mustache was a nice touch.

Were those guys active duty or were they retired/former service now working as PMCs? Some looked a little chunky to pass so much as a Navy PRT, let alone an SAS type program of work out.

The U.S. standard, in so far as their is one, is to be able to do a headshot at 25meters. Thus the institutional choice for a long time was a somewhat tinkered with 1911.

Army CID, general officers, and others also have M11s.

Edit to add, remember also that there are in efffect "two" types of SAS. There is the famous regular service sort, and then there is the "Territorial Army" sort, which is what Bear Grylls was. TA is more like the American National Guard.