to be honest your all wrong...
anything that makes the car sound LOUDER than the car does standard in the UK is illegal... yes a zonda is louder than a 1.2 clio but that isnt the point, taking the silencers off of a zonda to make it sound different is ILLEGAL as your making it louder than standard... so changing the exhaust on a clio to make it sound different is ok aslong as its no louder...
just like if you import a car and its modified and its under a certain age it will need a SVA test, where EVERYTHING has to be stock as the manufacturer intended it, there has to be a set of guidelines everything fits in to, as a mot on a stock 172 and a highly modified 172 will be exactly the same test but the cars are different so its not really the same car...
though not many mot places or police will pull you up on it! so do it and if u get hassle all u will have to do is stick the stock system back on and get a mot place to stamp the ticket and produce it at the police station.