I spoke to a couple of traffics about this, its only if people complain about the noise then you are breaching the peace, where the old bill take an interest is when they come and say hello and have a look and see you have a full 3" straight through system with a snakehead downpipe,full de-cat no resonators and a roger clark motorsport back box.
Then its emmisions check and MOT time again !
But im ok cos ive not got one of them LOL