Thanks for all your replies. I think my original message was a bit confusing.
I know that the airbag warning light is supposed to come on, and then go off. And I know that if it stays on, that something is wrong with the airbag system.
All I needed to know is whether the light staying on is an MOT failure? If not, then I'll put it through its MOT and then worry about fixing the problem at a later date.
My car has been off the road for a year and half now with starting problems. I've fixed everything and just want to get it MOT'd as soon as possible.