When deciding to modify your car its best to check the legal guidelines before you have any work done, the government have produced a downloadable document online that sets out the dos and don’ts quite clearly with images and descriptions to avoid any doubt.
If the police see that you have an illegal modification to your car they have the power to seize the vehicle, particularly if the modifications make the car unsafe.
One of the popular modifications that people make to their car is altering or fitting a new exhaust. A popular modification on this area, particularly by boy-racers is removing the silencer, this however IS illegal.
The reason that it’s illegal is partly that removing the silencer can cause noise pollution which can become a nuisance to neighbours, but also because removing the silencer shortens the exhaust meaning that poisonous fumes could collect beneath the car and enter the inside of the cab.