FWIW - i was sent to Leconfield to do HGV, I got as far as " the military are exempt from tacho law (at the time) and as such can exceed driving hrs."
That would be the "crown exemption" it allowed such things as driving hours, certain construction and use regulations and drivers age limits to be applied to suit service needs.
In my day in the RAF drivers hours were laid down in the JSP and were probably only exceeded during the build up to GW1 and Corporate, which if managed could be carried out safely.
In the lead up to GW1 a vehicle left our unit at 0730ish and was involved in a fatal RTC some 3 miles from base, a car had crossed onto the wrong side of the road and gone under the front of a fully loaded artic, the car driver had just completed a 12hr night shift and was driving home. We (the RAF) could demonstrate the Cpl MTD was fully rested and complying with the drivers hours regulations, as far as rest time and previous days hours.
As an aside, the vehicle was fully loaded with BL755 cbu's which sharpened the attention of the first police officer to attend.
PM