Exactly, it's not illegal to disable an airbag, nor is it illegal to alter a car so it would fail an MOT. However, in your scenario the owner would have a claim against the builder UNLESS it was made clear that the works they were carrying out under direct instruction do not pass current building regs. Can a building company even absolve themselves of that responsibility, as long as the work is not dangerous? I don't know.
Graham, if it's because your insurance won't cover it if it goes the way of the pear then that's a different kettle of fish. That's an insurance issue, not a legal one. I maintain though that if the work was done to a high standard that ticks all the boxes of the current regs and everything practicable was done, then there can be no liability.
Look at it this way, here's a few scenarios:
1. Switch badly installed by electrician, car sets itself on fire or airbag deploys when it shouldn't = Clear liability with the sparky as the regs were not followed
2. Switch installed correctly, but not in correct off position when rear-facing child seat used, car has an accident and kid is killed = No liability, work was done correctly and it was the user that got it wrong
That's how I see it.
I think that's what I said earlier:- "That might be the case Dan"