"Compliance with the Ultra Low Emission Zone (ULEZ) is based on the vehicle’s emissions performance at type-approval (i.e. when it is new). For a petrol vehicle, the requirement is that it has NOx emissions not exceeding 0.08g/km when measured over the type 1 emissions test at the time of type approval. That is equivalent to Euro 4 for a petrol car, but a number of Euro 3 petrol cars also meet that standard, although the emissions are not always recorded on the V5C (which we accept as proof). An alternative would be a type-approval certificate of conformity or a letter from the homologation department of the vehicle manufacturer to confirm the emissions performance.
If the vehicle is altered, it can no longer be assumed to have emissions as recorded at type-approval. The only way to prove compliance would be to have the vehicle tested on a chassis dynamometer over the type-approval type 1 test. This is nothing like MOT testing and not something that VOSA can verify. The vehicle would need to be tested at an emissions laboratory accredited for type-approval testing (such as MIRA or Millbrook for example)", (cost approximately £3,000 per vehicle).
"If you decided to follow this route, I would advise that you agree the details of the required testing with TfL in advance before entering any expenditure."
I'm still trying to find if there's any other way round this and I'll let you know what I find.