I had my HID's in for my MOT and was told for now it will pass but in 2012 they will be made illegal and will not pass the MOT. If you have HID's fitted to your car you need them to fit into very strict guidelines like projector lenses, auto leveling, headlamp washers etc. Unless they are manufacture fitted like on new cars such as audi, bmw etc then for one, it's gonna be hard to meet all the requirements and also you'll more than likely get pulled by the police alot and be questioned on your MOT.
In my opinion I love HID's and think standard bulbs look dated and yellow. But for all the hassle you can just spend a bit more money on better quality bulbs that can be just as effective.