The auto leveling (AL) system is attached to the axle and is designed, as mike says, to prevent the beam from blinding on coming traffic. There is some debate about the effectiveness of the system and many have fitted OEM xenons without AL- having the beam positioned in the correct place to prevent blinding + MOT failure.
The parts to retrofit fit the auto leveling system (2nd hand) are about £150, fitting is a pita but fairly straightforward from what I have seen in pics online. The headlamp washer kit can also be retrofitted, seems a bit more of a pita than the auto leveling from what I've read, but I'm no expert so it may be the other way round. I think you can add another £500+ to get these 2 bought and fitted.
I think the bottom line here is some people fit AL + headlamp washers some people don't. A lot of folks seem to think VOSA will crack down on this soon, how they will tell gawd only knows... I think it's aimed at aftermarket HID set ups without projectors where AL will never help the ***** beam pattern/ diffusion blinding people, than a well fitted set of OEM xenons (although it states that all vehicles with HID lighting should have washers and auto leveling as Mike rightly points out).
I guess it a case of you roll your dice mate, my new facelift xenons will neither have AL nor washers fitted until this is enforced and I have, I'm sorry mike... hopefully you don't get me driving behind you mate... lol, just kidding..... I know at least 2 others whom have xenons without AL + washers and I haven't been blinded by their lights nor noticed any dodgy beam pattern either.