You can fail on a loud exhaust but it's down to the testers oppinion if it is any louder than a standard exhaust on a bike of a similar type rather than an actual measurement. Most MOT testers aren't very oppinionated when it comes to Bullets.
I do play the game when I take my sportsbike in, I fit a baffle for MOT day and stick a bit of tape over the "not for road use" bit on the end can. If you can give them plausible deniability and it didn't rattle the windows and set off car alarms when you pulled up on it, most testers are ok. If asked later, they can say didn't see a not for road use and it had a baffle in when they tested it.