In the Britain, carrying a spare tyre is not a legal requirement. Whilst a spare tyre is not being used and is stowed away there is also no legal requirement for it to comply with normal tyre regulations. If a wheel or tyre needs changing, for instance if a puncture occurs, and the spare tyre is fitted, then it must comply by law. In the U.K, for passenger vehicles (not including motorcycles) that have under eight seats, and trailers and goods vehicles not exceeding 3,500kg maximum gross weight, the legal limit of tyre tread is 1.6mm. Ideally though, a tyre should be replaced if it has less than 3mm of tread. For motorcycles and vehicles larger than 3,500kg maximum gross weight, the legal requirement is 1mm. The following regular checks should be carried out: Tyre pressure. Depth of tyres. Check over tyres for damage including cracks, bulges and cuts. Check and examine spare tyre.
If you have one on the car it must be in full legal working order. As far as I know it`s not an mot fail if you havn`t got one.