I purchased one of these:
Its not a spare wheel, but it saves having one rattling around in the boot all the time. BMW do a similar inflation kit, but its over £100. I purchased this kit for £50 and it doubles up as tyre compressor all year around, it fits neatly under the boot floor next to the battery also. Ive not used the repair functionality in it yet, but you can buy replacement hoses/foam for after its been used. Pretty impressed with it so far and ive only used it to inflat tyres!
If i end up with tyre damage that one of these foam kits cannot even fix, i will just call the AA