I have my E36 M3 insured all year round, even when garaged over the winter, as its stored away from the house. Should probably have it on a "laid up" policy over the winter, but never bothered..........No MOT, no tax [no sorn either; £80 fine for being absentminded :-(].

Insurance companies aint interested if its taxed or mot'd, as long as you pay them.
If you drive the car illegally, insurance will be nul and void anyway, so they cant lose.

Hope this helps [kinda same as some have mentioned above already i suppose]