I think you need to find a new MOT station who understand that compelling a car to hit its emissions targets isn't the be all and end all, and are more flexible with the results.

Look for a local garage who deal with lots of classic cars and see if they either MOT themselves or find out who does their MOTs for them. For a car like the Williams it needs to go to a test centre where they're not going to knock seven bells out of the bodywork and are going to look at the mileage done and ask you about what you're going to do. It's not the same as testing a five year old VW that does 20k a year which is what 90% of test centres are doing 90% of the time; different criteria apply and you can't expect someone who does do that all the time to fit in with the needs of your car.