Volcano the law as always is a bit of a mish mash here, HID's and LED's are technically not illegal if the light unit was designed and tested with them fitted, however when you upgrade them the "E" conformity markings no longer apply as they are not "as" tested. This of course doesn't apply to new cars with LED's etc as the lights were tested with them fitted originally. I'm currently fitting DRL's to a spare set of heads which may or may not cross the line so I will wire them in thru one fuse which will be in my pocket come MOT time.
Airportable I currently have fitted what appears to be a unique O/S headlight designed and made with only 3 screws in the sidelight cover and nobody knows why, the N/S light is a replacement with the normal 4 screws. The photo shows that the little bit of moulding that sits in the middle of the cover doing nothing on 4 screw lights once was a continuation of the main body moulding.