Unless your car is already equipped with HID (Xenon) bulbs, then its not really an upgrade. Fitting the aftermarket bulbs to a refctor lense headlight gives a bad beam pattern, causing bleed and dazzles other road users. Not to mention that if you have HIDs you also need headlight washers (ok, some Halogen cars have these) but you also need the self leveling mechanism which you dont get with Halogens.
If your car has projector lenses, then it is possible to get a good beam pattern with HID bulbs, and you wont dazzle other road users, but for an MOT you still need the washers and self leveling system.
If you have halogens, then the best option is better halogen bulbs, like the nightbreakers suggested.
...fitting HIDs incorrectly just makes you look a t**t