On mk3 you should be fine without airbag because the original type approval for the model (for mk3 this would be around 1991 !!!) did not include airbags (for this reason you can remove it in picky places, say germany and then they will make a note on your car papers that are roughly equivalent to V5)
Generally if the type approval included airbags right from the beginning of production (i.e. after when they became mandatory to fit to all new cars) then you would get issues not having one. Though the UK is quite relaxed (or does not give a sxxx in other words) and in a small garage you may still get away with it.
Strictly speaking this would probably be considered a mod by insurance due to it being safety equipment (though I can't be sure as there are some really strange, no pointless rules in this country - was allowed a 10" kitchen knife on ferry but my safety knife with much shorter blade was confiscated because it had a locking blade...).
