R2, I do drive an LPG Skylaunch most weekends (and fill it when necessary). I agree that they give good launches and having driven a Supacat diesel winch as well must say I prefer the Skylaunch.
The MOD has had a policy for quite a few years of moving towards diesel/JP-8 fuels only, even the Royal Marines 'Rigid Raiders' use diesel outboards. The principle exceptions to this being training aircraft i.e. Tutors and Vigilants. Bringing in another diesel winch is a relatively simple matter, whereas an LPG one needs the infrastructure and all the HSE ball-ache that goes with it. In the post Haddon-Cave world, that would be a lot of extra work for each location.
One more thing; if the LPG storage tank is empty and someone forgets to order any, no-one glides at the weekend. If the tank is empty on a diesel winch, you could always fill it from jerry cans.