stick on number plates are not illegal just as long as yopu abide by the following
1. they are made from an authorised number plate maker to the correct text size and correct stroke factor spacing and has the company address in the small text so halfords or whoever.
2.rear number plates have to be lit up so that porky pig can see it
3. black/silver plates are for think pre - 74 cars only so if its made before or upto 1974 then black/silver is o.k. Anything after 1974 with black/silver plates are illegal