The Department of Transport are very clear on the matter and state:
"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".
Ever been blinded with incorrectly fitted HIDS ? The trouble with forums is that they're not really thinking about endangering the lives of other road users or other legalities. It's all about the look.
Those bulbs linked arent hids anyway, i'm running hids in mine at the minute and dont blind anyone due to the fact that we have lenses that have a cut off point, mine are actually less dazzling than modern audi/bmw/mercs.