not a law, but I believe in the insurance T&Cs it says you have to have a "reasonable interest" or some other such legal term in the vehicle - basically saying your insurance isn't valid if you do just this and take it out on a car you don't own etc..
Personally, i'd be looking to buy something crap, cheap and cheerful (a 500 quid fiesta or something) and put the policy on that - goal would be to find something that's as cheap as possible and cheap to insure as possible. Buy cheap enough and you shouldn't lose too much when you sell.