Technically under s5.4 of the current MOT Manual any airbag removal is a failure, however under para5 of the Introduction there is a caveat for extensively modified or converted vehicles i.e. rally cars or stretch limos. In this case, removal of an airbag is not a failure. However, you'd need to prove to the tester/DVSA that the car is indeed extensively modified for competition use. Modified road cars would not count.
However, it's all down to the tester of course...