I have only ever known one MoT testing staion to insist on taking the bike for a ride around the block, which was when he could check to see if the pipe was too loud.
When I took my bike there for an MoT I insisted they show me the regs that stipulated that the bike had to be ridden as part of the test...to which they had no answer. There was noooo way I was going to let anyone ride my bike.
If ever anyone is stopped by the plod for having a noisey exhaust, you can insist on a test being carried out at a MoT testing station at a date & time stipulated by you. If there's no markings like 'Not for Road Use' stamped on the exhaust, plod arn't MoT testers, & as a noise test isn't part of the MoT test, plod can do nothing.