o/p - your tyre bloke should be able to find you a one off rim. It does not need to be a BMW brand. As spare? Who cares. Get your bloke to get you a rim and use one of the old front tyre to put on that rim. An 18" rim here runs about 2oo bucks, surely your bloke should have an option at 140 sterling-ish.
Mind you.... a 20" tyre may not fit under the floor???