Rallyman:
As i said above, they are illegal unless you meet the three criteria:
Auto levelling (either the headlamps or the entire vehicle)
Headlamp washers
Type approved Headlamps.
The last one is the stickler, The manufacturer is the only one who can type approve the lamps and they'll only type approve the OE HID lamps as being fit for HID use.
You can do it legally using OEM Xenon lamps, and retrofitting the levelling and washing systems.
As you say, you might well pass MOT's and plod might think its fine, but should you one day be involved in an accident, and there is any inkling that the lights could have played a part, you WILL get boned.
Its the only reason i've not fitted them to mine, the cost of doing it properly is out of my budget at the moment.