OK, let’s start with me saying that as an aircraft commander and an AOC holder I also am aware of the rules as they apply to airliners. A Tutor is not an airliner, it is a light aircraft. You also use the word “Airport” in your post, which is not relevant to a Tutor operation from an RAF airfield.
The dispensation of which you speak refers to a statement made some years ago containing the words “A reduction in fire category does not, in itself, make a flight more dangerous”. I too have landed legally after fire cover has reduced below that required for the operation when the reduction has occurred after take off.
The Civil rules that apply to light aircraft operations (including light aircraft operating public transport flights to an AOC) do certainly allow a suitably equipped Land Rover to be the fire cover.
It has been a mystery to me for some time how the RAF manage to operate civil registered aircraft (Tutor) on what are almost certainly public transport flights (as not all (any?) of the aircraft commanders hold civil instructor’s ratings), without an AOC or civil professional licences. What the RAF do with a military aircraft is up to them, what they do with a civil aircraft surely comes under the CAA rules?