hid link
http://www.phoenixautobulbs.co.uk/tp/hid-kits-new-legislation-2012/
the link above has loads of info regarding the retro fitting of hids which as i though aren't road legal
here is a clip from the above site
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."
If you are found to be using lights which don't comply, in all likelihood, you may be reported to the Procurator Fiscal and may end up with a fine.
you also need to have a headlight self leveling system and a headlight washer system fitted as well,so its not just a straight forward swap over.