They're not strictly speaking "legal" but, with that said, my Scenic has gone thru the MOT with 6k units in without fuss for the past 2yrs and never once have I been stopped for dazzling people (Headlamp rig shows a good solidly focused beam pattern at the correct level without projector lenses). From Jan '12 the new MOT rules will specifically look at cars with "HID's" fitted for colour, intensity and to make sure headlamp washers and auto-levelling devices are working.
These rules makes no mention of checking for use of after market kits and I think its purely aimed at cars with factory fitted units and faulty position sensor so I don't forsee an issue unless you've got bright blue lamps burning the retinas of the tester out.
Mick