MT often sits idle on the weekends, when the cadets are busy. Just license the instructors appropriately, or pay ED pay to serving staff to drive for them. In the grand scheme of things, it would cost pennies.
The MT would also live in more secure accommodation. It's not that far of a stretch to suggest that anyone torching a minibus on an RAF station, Army barracks or RN shore establishment leaves themselves open to being shot in the process.
Whilst I agree with this in principle the practicalities of this is not that great, primarily due to the distance many parenting units are from their cadet squadrons.
When I was a cadet we were parented by RAF Uxbridge, yet I was a member of a squadron in Kent Wing, not the shortest of journeys to pick up rifles etc for shooting at Ash, let alone to pick up a vehicle. Then as a serviceman I was responsible for parented cadet units when I was at Headley Court, 48 squadrons (if memory serves me correct) yet our MT fleet consisted of a few cars and three minibuses, not a great deal to offer.
If local TA units could be used then such a scheme could theoretically become more practical but they parent Army Cadet units, which are more numerous, and white fleet wise, TA umits don't have a great deal of minibuses either (lots of Land Rovers and 4 Tonners though, both unable to be used to carry cadets).