Quite a few users over here have mentioned that it isn't legal to install HID kits on your car.
I'm not in the U.K. so I don't know if that is true or not. I checked a few sites where they mentioned that those kits are for exhibitions only and for road use. I'm not sure if that is right either.
In the U.S. they say it isn't legal to install a kit on a car that didn't have them from the factory in the first place.
I remember seeing an episode of Wheeler Dealers where Ed China installs a kit for a Lexus LS400. If it is illegal, why did they show that???
There are loads of topics all over this site!
To be legal, the whole system needs to be "Type Approved" for use with Xenon discharge lighting, so, the self levelling suspension, the light units and washers and even the wash system, so the bottles and piping!...
BUT... the HID retrofit kits are brilliant, as long as you get the H7R bulbs as they have a small portion blanked to prevent any beam scatter - standard H7's conversion kits, if adjusted are OK, as I drove 1000's of miles in the dark without a flash from anyone!! But have swapped as they were bright, too bright as some of the light still went where it was not needed!!
So it is your choice really - HID kits are illegal if being fitted into an existing halogen unit (on the roads anyway!), but if the right ones are fitted (H7R's) and adjusted correctly then they are fine, they will pass MOT's and the like - there are a lot of scare stories that MOT test stations are going to be failing cars with them in from 2011, this is rubbish! along with them failing cars that have none OEM maps on the ECU!! funny!!