I can only think that the "Garage or not Garage" question comes up so often because of the computer systems that the companies use. So that they can pay peanuts to barely trained monkeys on the phone those questions will all be in a Yes/No tick box, so removing any sort of initiative requirement from aforementioned neo-simian types.
Similar to the no house number in the mess, trying to get quotes for BFPO can be difficult, particularly for the emerald toilet, where no one seems to want to insure anyway.
ComJam, did you try BMW's own insurance, heard they can be pretty competitive. I would recommend Privilege, have renewed with them for a couple of years, as their premiums have actually been going down!