Cars with them fitted as std have different lens shapes and materials. Apparently (From another forum) the beam pattern is too bight in places for the machines to test and if they can't test it they can't pass it. The solution was revert back to normal lights for the MOT.
I tested and lined up both cars at a friends garage. He does MOTs and has the relevant beam height tester.
Basically you need a clear line at the top of the beam to set the angle. Both of my cars where fine - I mentioned about the legality thing and he said they would both pass
and MOT as he can see a clear line at the top of the beam.Some
cars however have lenses which wont show this line with HID bulbs, and some HID bulbs are of such a poor quality they themselves are unable to produce the line....