I have a theory; The fact that the USAAF used Follow Me vehicles, and the RAF didn't is down to the different ways the US had in dealing with situations compared to a more reserved ways of us British. Being the arsenal of democracy the USA had an almost endless supply of vehicles, so I image the CO of any USAAF base would just say "Get one of these Jeeps marked up and in use right away" and seeing as every unit was invariably equipped with Jeeps, Dodges and GMCs it didn't really matter to take one for this role.
Whereas the RAF having to make do with all manner of vehicle types, none of which would make an ideal Follow Me vehicle. Obviously the specialist refueller trucks, crash tenders, ambulances, cranes, and bomb tractors would be out of the question for such a Follow Me role, and that would leave just GS vehicles in 8cwt, 15cwt, 30cwt and 3 ton capacity. But the fact is that in WW2 the RAF could not afford to take one of these vehicles and use it for just one purpose only.
Also I'd image the RAF CO saying you can't mark up that Tilly as a Follow Me vehicle as I need it to visit Group HQ (and the pub & girlfriend etc.)