It is perfectly legal to carry whatever tyres and wheels you like on your vehicle - round, square, flat, inflated. The 'spare' wheel is no longer inspected at the MOT test. If you want to keep an odd collection of non-matching wheels in the back of your car then it is up to you. The law only comes into it when you USE a wheel by actually fitting it to the vehicle. At that point the vehicle construction and use regulations come into play. Basically, amongst several other things, they state that the wheels on either side of an axle MUST be of the SAME size and type. Drivers whose vehicle came with one of those pathetic little 'space-saver' wheels, which are legal in some other coutries, get away with using them by the cops 'turning a blind eye'. A similar situation applies to several other weird foreign ideas on some new cars - for example, the handbrake on some new BMWs which is not purely mechanical.