Hi Tom - spotted this on the "tyresmoke forum" and although dated 2004 it may well still be the situation from the DOT. Seeink below but in case the link don't work I have copied its comtent below - I appreciate it is a bit wordy to say the least but seems to answer the question.
Tyresmoke.Net: Xenon Headlights
The situation for Gas Discharge (HID High Intensity Discharge) (commonly known as Xenon) headlamps is complex.
I attach links to the Road Vehicle Lighting Regulations 1989 which regulate the situation in the UK.
Under these Regulations Xenon headlamps are not mentioned and therefore they are not permitted according to the strict letter of the law.
However you will be well aware that new vehicles have Xenon 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 Xenon headlamps which are tested on a rig in a laboratory) and ECE Regulation 48 (Lighting Installation on the vehicle).
For the aftermarket, a used vehicle cannot obtain type approval because it is only applicable for new vehicles. However we feel that saying "Xenon is banned in the aftermarket" would not be reasonable. Instead we should make analogies with new vehicles. It would be reasonable to require Xenon in the aftermarket to meet the same safety standards as on new vehicles. The same level of safety should apply.
Therefore a Xenon headlamp sold in the aftermarket 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.
Hope that helps.
We are aware of kit available in the aftermarket where a Xenon "burner" or bulb is fitted inside a headlamp designed for use with conventional Halogen filament bulbs. The burner is fitted with a "bayonet" type fitting so it fits where the Halogen bulb should fit. This is not legal and the vendor, the person who mounts it on the vehicle and the person who drives the vehicle are all committing an offence. (Which also means that he is invalidating the insurance.)
The reason for this is that headlamps and bulbs are made to tight tolerances and the wrong type of bulb will cause an incorrect beam pattern to be emitted, which could cause dazzle and discomfort to other drivers