There is no noise limit as such for the annual UK motorcycle MOT in terms of db limits. It is down to the tester's discression so while one may pass a certain exhaust system another may fail it & the third may just issue an "advisory". That is providing the exhaust cans or pipe do not have "NOT FOR ROAD USE", "RACE USE ONLY" or any similar wording stamped on them. If you have what you might think is a noisey exhaust you need to find a sympathetic tester.
Some MOT/dealer test bays are now loathe to stray beyond the line. Big fine if door stopped by them. (Inspectorate) I know the chances are slim but now lockdown is easing I bet some are chomping at the bit to get out and start checking.
My MOT guy did that for me on my air-cooled monster but he's very sympathetic - he owns a 888, 916, RC45 etc etc