Illegal technically, but I've driven all over the place with one of them on and never got a pull.. if you ever did I imagine you'd just get a producer or something for it. Plus, my dad managed to avoid a bus lane camera ticket with one of those over his number plate so it's a pretty useful accessory

Tip though, if it's the type with the 'suckers' on the bottom, put some cloth between them and the car as they can scratch the paintwork.