As far as the MOT is concerned it depends what the tester would deem "clearly unreasonable"
Method of inspection.
4. During all parts of the emissions test relevant for the vehicle, assess subjectively the effectiveness of the silencer in reducing exhaust noise.
Note: For vehicles not subject to an emissions test, the engine should be revved to around 2500rpm or approximately half maximum engine speed if this is lower.
Reason for rejection.
4. A silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a standard silencer in average condition.
