To recap.
MOT rules.
The spare tyre is not a testable item.
If the spare is missing as long as you have no other failures, the car will pass.
If the spare is defective (bald tyre or flat) the MOT tester should give you a VT32 (Advisory form), which advises you of the problem.
The spare wheel if it's suspended under the car on a carrier must be secure.
The Law.
It is not against the law to drive a car without a spare wheel.
http://www.askthe.police.uk/content/Q602.htm
However if you have a spare wheel in your car, it must legal.
http://www.askthe.police.uk/content/Q405.htm