It would fail an MOT. We had an air-bag warning light on on my car (wanted £1500 to find out what the problem was). Luckily I decided to look and see if my car had been recalled at all and it had - so Chrysler had to check it and sort it out for free.
IMO if a vehicle is roadworthy without an air bag, a vehicle with one but with it's light permanently lit ought not change the status. Unless the expectation is that it will go off 'without good cause' unexpectedly and cause an accident, which is presumably unlikely.
it is illegal for anyone to issue a mot if there are any warning lights on permanently.this is because the system is informing you of a fault and a possible safety fault.any mot station that passes a vehicle with faults is most likely to be fined and struck off.if the poster who has been running his car with the airbag light on for 3 years has not had it corrected in that time and has had mot issued with the fault then he can be prosecuted along with the mot testing centre plus the car can be impounded