The vast majority of Crew Cabs can be driven on a normal car licence because they have 8 or less passenger seats and are less than 3500kg GVM.
I drive hired MoD ones all the time on my B cat licence and never had a problem. Unless there is something special inside the particular van you are looking at you should be OK.
If you are still not sure, ask the seller to show you this plate. You can normally see it when you open the passenger side door:
If the top number is 3500kg or less, you are fine.
For a squadron owned vehicle which has a permit 19, you can drive anything up to 16 passenger seats, as long as it is 3500kg or below. Some Ford minibuses with 14 seats can be driven on a car licence with a permit 19.
Hope that helps!