The law on aftermarket exhausts

[Adam 180 V6]

Hi, earlier on i was pulled by the police and he told me my exhaust was illegal, when i asked whats illegal about it he said to me "any exhaust thats not made by the vehicles manufacturer is illegal" the exhaust is a long life stainless steel exhaust and its not really loud it just gives a nice tone, its passed years of mot`s with it on and never a problem. I said to him i cant get a manufacturers exhaust from a manufacturer which doesnt exist anymore, (well it does but it aint the same MG) Anyway he said if he sees me again he will be fining me for the exhaust, Whats the law on after market exhaust as i believe hes just wanting to be an arse and hand fines out.