This is somethign talked about a lot on several other forums. Ironically HID lights are illegal under UK law full stop. It is only EU law that allows them to be used in the UK.
http://www.danielsternlighting.com/tech ... sions.html
This is a great link for information on it.
For cars the legislation is quite clear that almost every retro fitted HID kit in the UK is illegal. For bikes it appears to be a grey area but anyhow.
A proper HID(which is what I assume you mean by Xenon) uses a projector housing rather than a reflector housing. In the early days of these retro-hid kits you had a hacked up bulb that fitted in your standard headlight.
This was essentially crap.. HID's need a projector housing to produce a correct beam pattern so you got a messed up crap of light.. people thought it was better as it lit more of the immediate area (much like fog lights) but this is actually a bad thing as your eyes adjust to the immediate area rather than the long distance and you can miss hazards.
No matter what anybody says, sticking an HID bulb inside a normal reflector housing will never produce a correct beam pattern
Newer retro fit kits use a proper projector housing, you cut up your existing lamp and install the projector housing into them.
Some of the projector housings are not e-marked and some are, fitting them inside another headlight is also grey.
At least by using a projector unit hacked into another casing you will be getting a decent beam pattern that wont blind other drivers. I wouldnt like to argue one out in an RTA though, especially if the other driver claims you blinded them.
I have seen some impressive retrofitted HID projector houses in OEM headlamp casings but I just stick to Phillips X Treme bulbs in all my bikes, knowing that there totally legal, brighter than normal h4s, cheaper than hid kit and knowing that i wont have any problems with them, visibility isnt a problem either.
Each to their own.. if you do want to do it though, be aware its all a grey area so you do run the risk of getting shafted by a policeman, even if they cant do you for illegal xenon you can still get a 60.00gbp fine for bad headlamp adjustment or whatever its called, plus insurance nightmare.
The best beam pattern you will get is by fitting a projector housing into your headlight, it is correctly focussed and you should be able to adjust it.
http://www.blindinghid.com/hid-conversi ... rojectors/
Plenty of websites like this about.
If your thinking of sticking a HID bulb soldered to a H4 plug into your normal headlight, honestly forget it. it might be brighter and look better but youll have worse overall vision, blind other drivers and face a nightmare if anything bad happens.
Hope that helped somehow