re David's answer ~The spare on my car (like many others) is located under the floor on the inside of the boot, so if it was missing it wouldn't make any dfference.
I put the car in for it's first MOT last year (December) and the spare was wrecked. I'd had a puncture over the Christmas break and the tyre had partly shredded. The car had to be MOT'd before I'd had a chance to get a new tyre fitted on the spare but it still passed. The testing station said as it wasnt usable it didn't matter, and even if it was bald it would only count if it were to be 'in contact with the road' i.e. fitted on the car. It would then become part of the test criteria.