The HAC do not use the Ceremonial vehicles for combat roles these vehicles with the Ceremonial 105mm field guns are stored in air conditioning garages and only used on Ceremonial Duty’s All other vehicles are what they call GS vehicles which they have approximately 15,these are landrover 110s a couple of wolfs and 4Ton GS, they have specified vehicles for thier role as surveillance and Reconnaissance roles and the HAC Role is Surveillance and Target Acquisition, the vehicle you have is probably from the “Pool” and is most likely use by many Different units in the British Army, The HAC is the oldest military corps in the world and was chartered by King Henry V111
I hope this is useful