MOT testing stations do not need to have noise meters. This has been going on in the motorcycle world for ages. ( I have had to change the race pipes on my Ducati 748 SPS several times before)
Just an extract from the VOSA, I know it is applicable to M/c's, however I remember reading somewhere that this had also become applicable to cars, I hope my memory is wrong, but it is time to get friendly with your local station.
The instructions will include an instruction to assess the exhaust noise with the engine
being revved, as currently it is assessed at idle, also to include a reference to a
standard silencer in the Reason for Rejection, as in the Motorcycle Manual. As
stated in the Method of Inspection wording, the test is subjective, and therefore relies
on the tester s judgement. MOT testing stations are not required (or permitted) to
use noise meters assess the levels of exhaust noise. However, as you intimate in
your e-mail, an MOT is issued to a vehicle and only confirms that it met the minimum
standard required at the time of test