Rear number plate

[flyerncle]

(2) Subject to paragraph (3), a registration plate shall be fixed vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practical at the front and rear of every motor vehicle in such manner that the characters of the registration mark shall, at all
times, be visible from the front or the rear of the motor vehicle..

I think there was an exemption for the front one as there is not much to attach it too, and there does not seem to be a height stated apart from it must not be above a certain height.