All I can tell you is my experience. I was not sure what to do when I built my first 250 California. I understood that the only way an MOT tester can fail the car is if the airbag light is illuminated and showing an fault, or does not illuminate at all, which also shows a fault. If it comes on with the ignition check, and then goes off, he has to assume that all is working correctly. He cannot test the airbags themselves, not would he be prepared to or even allowed to disassemble the car to check for an airbag presence. So the choice was do I not fit an airbag light, or do I fit a light that does what an airbag light does. I'd heard of one builder who had an MOT fail because "there was no airbag light, and Z3's have airbag lights so if its built on a Z3 it needs the light". So the car went back 2 days later with a fake airbag light and passed the MOT.
So I did the same and fitted a fake airbag light (in my case it was wired into the ABS light which came on and went off after a few seconds when the ignition was turned on. Obviously if you ever have a problem with the ABS then it will also appear to be an airbag problem, but I'd get the ABS fixed as this is far more important that the airbags. If I can't stop, I'll hit something hard and the airbags would be useful. If I can stop, I'd hope I never get into a situation where the airbags would have been required.
I am happy to take the small risk of not having airbags. I was driving cars in the 70's and 80's without any of this stuff.
I had the car for 4 years before selling it, and it passed the MOT without issues every time. The testers are usually so exited to test something different they don't even think about the airbags as long as the light shows a functioning system.
Currently building my second 250 California and this time I have bought a timer for the airbag light from eBay
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