OK ... with regard to aftermarket silencers, they can't have "not for road use" stamped on them and can't be "no louder" than a standard silencer as a rule.
This is an extract from the MOT rules...
The exhaust system and silencer should be in such condition, or of such a type, that the noise emitted from the motorcycle is not clearly unreasonably above the level expected from a similar motorcycle with a standard silencer in average condition
However, it's really down to the MOT testers discretion as he/she/they/it are supposed to know what a standard silencer should sound like.
Personally, If a silencer isn't tearing my ear drums out I'll Pass it with an advisory that its "slightly" loud.
FYI ... if anyone comes to me with an aftermarket silencer flashing a certificate (and they have) I disregard it , it's means nothing.