Afaik, insurance doesnt always cover you on private land anyway (ive heard stories for instance of people having knocks in a supermarket carpark and the insurance not covering them because they are on private land).

You don't need tax to use a vehicle on private land, the same as you dont need tax to keep it on you're drive.

Im not 100% on the MOT, but if its at you're own risk on private land, then what can they say, the MOT is for public road use, how many of these stock cars have an MOT lol.

Id be more inclined to believe he was on council property or there was some other variable at play in the polices decision to crush it.