I think the biggest problem area for people with modified cars is going to be air bags. A lot of things are open to interpretation but it seems pretty clear on the air bag rule. "The vehicle will fail the test if any air bag fitted as original equipment is missing or defective"

There's a lot of people with road legal 'track' cars that gut them out. Surely it doesn't get anymore black and white than that? If your 1999 impreza rollls in to the mot test with a plastic dash and no passenger or drivers air bags then it will fail.

It also states a fail if the car is missing a safety belt pre tensioner that was factory fitted.