To be legal you must fit a complete headlight unit for HID system, which must be E Marked. The headlights must have auto levelling and washers.
THE LAW
Aftermarket HID headlamps: December 2006
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.
Link to THE LAW
http://www.dft.gov.uk/pgr/roadsafety/drs/hidheadlamps
I am surprised that you did not find upgraded halogen bulbs a significant improvement. I used Phillips upgrades on mine - before the availability of the latest 25% and more versions - and judged them a complete success. My A3 and A4 have been fitted with Osram Super Stars (or whatever they are called) and are sound. Perhaps an alignment heck and adjustment is in order?
