In the UK they actually have not been passed as legal for road use. Only the fact that they are legal according to EU legislation means they can be used in the UK.
VOSA's stance based on EU regs is that to avoid scatter and vertical misalignment cars using them must have headlight washers and either lens or rear axle auto levelling. All new cars sold in Europe that come with HID lights must meet the equivalent EU regulation.
Therefore, fitting aftermarket HIDs is technically in breach of these regulations. However, as long as you have told your insurance and the car is free of any defects which are picked up on the MOT, then your car is road legal. My car passed the MOT at a Honda main dealer a week after fitting them.
So the only trouble you are going to get is if a bored and eagle-eyed police officer decides to start prodding about, and as has been said, there are that many cars with them it would be difficult to notice.
It's in the same league as using a decat on your car, some cars will pass the MOT with the decat. Will you ever get pulled by the police for it? No.
Good point about the colour too. Mine are 6000k. On my phone at the moment so can't post pics but they are in my build thread on my Sig, will edit later and add pic.
Edit:
Pics!