Basically, you need to change the whole headlamp if you want to be legal:
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. -
Department for Transport (2006)
This is taken from the Department of Transport's information Sheet on aftermarket HID headllamps and can be read in full here:
http://webarchive.nationalarchives....ormation-sheets/aftermarket-hid-headlamps.pdf