The wording of the new MOT rules is a bit ambiguous to be honest, and I know HID sellers will say it's perfectly legal to fit their kits. But they would, and they're hardly going to tell people that they're flogging something that's a bit sketchy on the legality front, and dazzles other road users.
I hate them (the kits, that is, not the HID sellers)
But you'd probably find on something like, say, a Passat where it's possible that it could've had them as standard that they'd be less likely to look out for an aftermarket kit. Rock up with a Polo that never would've had them, and even has the wrong headlamps, and you're asking for problems IMHO.
Though like all things it depends on your MOT tester. Jobsworth testers willl probably take one look, spot the aftermarket ballasts and fail the car straight away. But then they're easy to take out for the MOT.
Personally I'm quite happy they're clamping down. I do a lot of night time driving and I hate getting dazzled by naff aftermarket HID systems in cars which blatantly shouldn't have them.