all I could find out is as follows mate:
"As far as the law is concerned, the normal requirements for lighting (in Britain) are that there should be two bright white front lights, two red tail lights, two red stop lights (if fitted - originally or later),
a number plate light, and some form of indicator system (two arm-type semaphores or four flashing orange lights - again, if fitted as originally or later). All cars must have two rear red reflectors fitted, even if not supplied originally. There are some exemptions to this list, albeit for the earlier cars. If your car was manufactured prior to 31st January 1931, you do not have to have two rear side lights. M.G.s, even from the outset, were always fitted with a complete compliment of lights, except for directional indicators."
It is for cars (obviously) but I guess the same applies for number plates regardless of vehicle type, ie, they must have a light. It dont say how bright