There's ample vagueness to converse with your MOT inspector here. I know as I've been through my fair share of MOT issues due to faulty headlamp levelling system with factory fitted, self levelling HIDs.
My interpretation of this is: if the car does not have self levelling headlamps or headlamp washers installed, it can still legitimately run HID lamps without issue, providing they're focussed/angled appropriately. Therefore retro fitting HIDs to a car without self levelling and/or washers isn't an issue in my opinion. If the headlamps have projector lenses, the angling and light dispersion normally isn't too much of an issue. Older headlamp housings you'll be blinding everyone, probably unable to focus the beam within legal limits.
Furthermore, is your car lowered? If so, it can be very hard to bring about headlamp movement significant enough for the test (cue the benefit of the doubt). For example, my car is lowered and they gave me the benefit of the doubt one year on account of that they couldn't get the rear to drop enough with the weight of the mechanic standing in the boot (due to stiff springs). Although they knew one of my headlamp adjustors was seized, but the beam angle was good and they were helping me out on the proviso I sorted it for next year, which I did.
So if you don't have factory fitted HIDs and want to run HIDs, it's my opinion, you should be ok to do so. If your MOT inspector gets funny, show him the guidelines online. "Where such systems are fitted" in other words, the system doesn't have to be fitted, but if they are, they must work (as was the case for me)! It's the spare tyre thing. You don't need one, but if you have one, it must have sufficient treat etc...