Biggest problem is that all legal E marked headlights are actually coded. This is not a serial number but rather relates to its approved application. This includes down to its bulb type. Any headlight approved for halogen bulbs can not be also approved for HID bulbs. This is why all retrofit HID bulbs are illegal to use on the road. You actually have to swap the hole headlight for one approved for HID and takes a standard HID bulb such as a D2S. The UK Governments Department For Transport Fact sheet on the subject http://www.dft.gov.uk/pdf/pgr/roadsafety/drs/hidheadlamps
If you do read the DFT fact sheet don't make the common mistake of thinking this is opinion. There is only one part that is opinion. Strictly speaking any car that did not come with HIDs can not legally have them fitted (that part is fact) the part that is the DFT opinion is that they think you should be allowed to fit HIDs as long as they are to the same standards as that fitted to a new car.
