The VOSA, Vehicle operator standards authority normally look after lorries and busses, i work in the industry ad beleive me, they are the gods, the can also deal with cars and vans, there is a fine called a PG9. Prohibition. this can be delayed or immediate, if it is an immediate it will only get towed by you, you cant pass go or drive until re tested. it is most certainly a PG9. vosa have got more power than the police when it comes to the road and what is on it.
That is FACTUALLY how it stands,
i got a decat though, bolt in bolt out, test, no test. main reason you would get cought would be the noise level when you nailing it past the traffice car, answer = do not nail it past them.