From another site. referenced to department if transport.
"In the Department's view it is not legal to sell or use after market HID lighting kits, for converting conventional Halogen headlamps to HID Xenon. If a customer wants to convert his vehicle to Xenon HID he must purchase completely new Xenon HID headlamps. The reason for this is that the existing lens and reflector are designed around a Halogen filament bulb, working to very precise tolerances. If one places a HID "burner" (bulb) in the headlamp, the beam pattern will not be correct, there will be glare in some places and not enough light in other places within the beam pattern.
The following is the legal rationale:
The Road Vehicle Lighting Regulations 1989 regulate the situation in the UK.
Under these Regulations, HID/Gas Discharge/Xenon headlamps are not mentioned and therefore they are not permitted according to the strict letter of the law.
However new vehicles have HID headlamps. This is because they comply to European type approval Regulations. The UK cannot refuse to register a vehicle with a European type approval. These are to ECE Regulation 98 (for the HID headlamps which are tested on a rig in a laboratory) and ECE Regulation 48 (Lighting Installation on the vehicle).
For the after market, a used vehicle cannot obtain type approval because it is only applicable for new vehicles. However we feel that saying "HID is banned in the after market" would not be reasonable. Instead we should make analogies with new vehicles. It would be reasonable to require HID in the after market to meet the same safety standards as on new vehicles. The same level of safety should apply.
Therefore a HID headlamp unit sold in the after market should:
1. be type approved to ECE Regulation 98 as a component.
2. when fitted to the vehicle should enable ECE Regulation 48 to be complied with (although no government inspection will take place).
3. Comply with RVLR as far as "use" is concerned.
In practice this means:
1. The headlamp unit (outer lens, reflector, bulb) shall be type approved to ECE 98 and be "e-marked" to demonstrate this. That can only be done by the headlamp supplier - Hella, Valeo etc. who must test the headlamp in an independent laboratory.
2. Once fitted to the vehicle it must have headlamp cleaning and self-levelling (which can be for the headlamp or can be in the vehicle suspension - some expensive estate cars have "self-levelling suspension" and that is adequate). Also the dipped beam must stay on with the main beam.
3. The headlamp must be maintained in good working order, kept clean, and aligned/adjusted correctly like any other headlamp.
Under the Road Traffic Act 1988 it is an offence to supply, fit or use vehicle parts which are not legal.
In summary it is not permitted to convert an existing halogen headlamp unit for use with HID bulbs. The entire headlamp unit must be replaced with one designed and approved for use with HID bulbs and it must be installed in accordance with the rules stated above."
The above seems to be a precursor to the new MOT criteria slated to come into existance on New Years Eve 2011 (for 2012), this is from the MOT Testers VOSA bullitin explaining the new EU MOT regulations which were ratified this year, and includes amongst other things, the testing of wiring harnesses, the testing for illegal HID kits and, wait for it, chipped ECU's, whatever that means The article is quite long, but here is a small extract .....
"As far as changes to the test content are concerned, VOSA has already been analysing the requirements of the new Directive and working out how to implement them. We started this earlier in the year by talking with representatives of the MOT trade at our regular Trade User Group and VTS Council meetings. Both VOSA and the Department for Transport (DfT) are keen to ensure that any changes to the test are introduced in as practical a way as possible, keeping the burden on the trade to a minimum and ideally keeping the changes cost neutral.
In many cases, the changes shouldn’t necessarily lead to an increase in average test times. A good example is the malfunction indicator lamps on the dashboard that indicate defective electronic power steering, electronic stability control and secondary restraint systems. Testers already check the dashboard for other lamps, so no extra time would be required for this addition to the test.
Electrical wiring and batteries are now included in the test’s scope, but testers already check the vehicle structure where wiring is secured – often along the same routes as other testable items, such as brake pipes in the engine compartment. So again, this doesn’t look like an additional burden on the tester. In the pre-computerisation days, testers often (wrongly) failed vehicles for insecure batteries, so they must have been looking at them then! Now, it means that when we implement the new Directive, vehicles can legitimately fail for battery insecurity, for no extra tester effort.Other items – such as headlamp bulb and unit incompatibility, headlamp levelling devices and illegal engine ‘chipping’ – will need further thought before we can get a workable solution for MOT stations."