our local MOT shop said that Space Savers are OK as the manufacturers have got them approved for the specific vehicle, they are marked as for temporary use only and speed limited etc.
however a copper i know has said that what tends to happen with these "trends" is that other, normally older cars "can" get away with something similar. For example it was always illegal to have non symetrically fitted, front (why front only i dont know) number plates until alfa romeo got the UK law changed to allow this on the 156, but i'm sure we've all seen this before on Fronty's both before and after the change.
matey says it depends what you were doing when stopped by the fuzz, nothing too drastic and being polite then probably ownt even notice. drunk and try and do a runner then evry little thing will be checked and held against you

you've also got the other factor of if the wheel/tyre is obvioulsy different then questions may be asked, similar then others may not notice. what you must have is same size on each axle both legally and for safety, different sizes on different axles ok along with only one spare (for mot and law) but you can only fit to the axle with same size. its no longer a requirement to have a spare at all let alone right size but if you do then it must be legal tread.
different sizes on each axle may be ok depending on the car and what its for etc but i guess that even off road, if different diameters on a Fronty then wind up may occur more frequently(?).
personally i'd always try for same sizes across each wheel inc. spare on my fronty but if i got stuck on a wet, windy muddy day somewhere id fit anything to get me home!!