Simply put, their illegal.
Thing is, police really got better things to do then screw you over for lights unless their an absolute cock. MOTS are usually passable aswell as their more concerned with the alignment of your headlights than the bulbs itself ... you could easily just throw in normal H7 Yellow Halogens just to pass the MOT and take them out as soon as you get the certificate right in front of their face; knowing this fact, MOT Testers usually just shrug their shoulders when it comes to HIDs.
Thats from my experience anyway ...