Not totally true. My Ferret is built before 1960, therefore is MOT exempt on that clause. Look for one with BA, BB, CA or CC registration numbers. Later vehicles DA, DC, DD, EA etc will not qualify. Our Fox is MOT exempt on a different clause. It's sole purpose is to transport a gun, therefore it becomes a gun tractor, and is exempt from MOT testing. There are other rumours going around about them being 4 wheel drive and impossible to test, etc etc. I've never seen the 'armoured car' one before though. There are ways and means to make them exempt. The guy at our local DVLA office said he tries to get as many of these 'unusual' vehicles exempt as possible, as it just makes life easier for everyone.
Chris