The point is that most really loud cars are made to be loud. And usually equipped with a way to quickly undo that, if needed.

I’ve seen a TV report on some policemen hunting “tuned” cars. They were following an obnoxiously loud car in an unmarked police car, stopped them, and took measurements - suddenly, the car was “just normal”. But they knew what they heard, and the measurements they had taken from a distance had been way louder than the measurements taken at a defined distance from the exhaust, so they impounded the car for further investigation. And found a switch in the glove compartment that changed the car from “normal” to “loud”.

You’ve got to catch them red-handed. As long as they can disable or just quickly undo something like that before a MOT , it won’t get a single idiot and his car off the road.