Jassy - parts departments can and do sell these - the reason being you can do exactly what my parents did - move to france and take their right hand drive car with them, and eventually settle down and replace their headlights with ones that beam in the correct direction - the point being they were replacing UK HID lights with Euro HID lights for driving on the other side of the road, and the other reason being someone with a HID car may get a stone through their lights so need to replace them.
If you as a UK customer replace your non HID lights with HID lights and do not add headlight washers, you will be driving uninsured (as your insurer will refuse to back you up in the event of an accident, when their inspector comes and looks at your vehicle and finds that it is not roadworthy due to having illegal headlights) and you will fail your MOT at MOT time. As someone who has been through several motoring law courses recently I can tell you this for fact.
You should also find that if your insurers are up to speed they will refuse to insure you when you phone and tell them you have changed your lights to HID if you have not also added washers, or they may offer to insure you for daylight driving only as an alternative...
Just because something is available for sale it does not mean that it is legal to use it.