Just found this on web during my investigations -
www.dft.gov.uk › ... › Road safety › Driver and rider safety - Cached - SimilarHID Dipped Headlights :
Here's just the start of it - '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.'
It goes on with further why's and wherefores - any views on it?
Basically if you want HIDs your headlight casings must be approved and stamped as suitable for such use by the manufacturer.