What, by law, is classed as 'having an exhaust'...?

If you have a 4-1 manifold, which basically has a slight downturn to the end of manifold under car, that is in my opinion still an exhaust?

Having nothing bolted to the head on the exhaust side is not. And i haven't heard of any official aftermarket exhaust noise limits either.

As long as you dont trip pedestrians up then im pretty sure it wont make any difference. As long as they can get to it with the probe for MOT time lol