I have my Alter II (a swb station wagon) insured with Adrian Flux on a 5000 mile a year policy, parked on the driveway for a few hundred notes I seem to remember.. I also tried NFU and one other that knew what the vehicle was but wanted almost a grand.
I have another two specialist cars insured with Adrian Flux and would recommend them, always found a good service and quick at responding to questions.
Where abouts are you based? There is another UMMbulance in devon that is converted to a rather smart camper.