You can buy special number plate double sided sticky tape from halfords and it works fine, just make sure you buy enough(might need two packs), they were located at the parts desk in my local halfords.

I used them to stick a front number plate on my old golf, seemed to stick firmly and didnt come off just make sure to stick it on when its not raining and when your not going to be using the car for a good few hours, and make sure you give the sticking area a good clean(alcohol wipes come with the stickers), then it should stick well.