its not a crime but a civil matter that does not come up on the mot but will fail a road side check. it wont give you a criminal record but will get you a fine.
mine was possitioned on the corner of the bumper, but was told to move it to the middle or to a more forward facing possition. this is so that ANPR's and Lazers can pick them up. (another reason a lazer gun cannot pick up a motorbike) i digress..
a number plate should be possitioned in clear view and of correct size, facing the front of the vehicle.
i have seen trucks with them on the roof. so as long as they point to the front. no probs at all.
and no., bonnet stickers arent legal unless the vehicle comes under a Qplate catagory of kit car.