All new motorcycles sold in Europe must have European Whole Vehicle Type Approval, it is not possible to obtain this approval for a motorcycle fitted with HID lamps. This is because there are a number of technical issues that have not been overcome for motorcycles.
When passenger car manufacturers obtain European Type Approval for vehicles fitted with HID lamps they are required to fit automatic headlamp levelling and automatic headlamp cleaners. The reason for this is the need to reduce glare to oncoming drivers due to the high luminous flux of these lamps. Automatic levelling ensures that the lights remain aimed at the correct angle downwards under all vehicle loading conditions. Automatic cleaners, designed to operate the first time the windscreen washers are used when the lights are switched on, are necessary because dirty headlamps are a major cause of glare. Putting these two devices on a motorcycle posses significant problems for the manufacturers. Automatic headlamp levelling requires the addition of some form of electric motor to move the lamp correctly and a sensor to automatically adjust the light. This adds significant expense to the cost of the motorcycle. Headlamp cleaning also adds expense and is difficult to apply since the most likely result is that the rider will be covered in cleaning fluid when the cleaners are activated. Further problems arise on motorcycles due to their ability to lean over which may also result in increased glare from the lamp.
European Whole Vehicle Type Approval is only required for new vehicles. Once the motorcycle has been registered for use on the road in the UK it is then required to comply with the Road Vehicle Lighting Regulations (RVLR). As you point out, RVLR does not require lamps to be marked to show approval with the appropriate European Directive and there is no specific prohibition from the use of HID. However, RVLR does require that lamps should not be used so as to cause undue dazzle or discomfort to other persons using the road, Regulation 27. For this reason I do not believe it would be acceptable to retrofit HID lamps to a motorcycle unless measures are put in place to reduce the possibility of glare. I suggest as a minimum these should be as stated above for passenger cars but additional measures may also be necessary.
The above information is provided by way of advice. Only the courts can provide an authoritative interpretation of regulations. It is the responsibility of the vehicle owner/driver to ensure that the vehicle at all times, when used on the public roads, complies with the appropriate legislation.
Don't worry so much
I don't think the regs apply to motorcycles. The DoT factsheet states that "The headlamp unit (outer lens, reflector, bulb) shall be type approved to ECE 98 and be "e-marked" to demonstrate this."
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