Everything on a car is illegal if it isn't done correctly.
Tyres on cars are illegal if they aren't installed properly or have no tread etc but it doesn't mean we all shouldn't be putting them on our cars.
The same goes for HID's.
As long as it is installed correctly and abides by the rules and regulations then it's fine.
The problem we have is boy racers and inexperienced 'modders' who think it's as easy as going to your local Halfords and buying a cheap 'universal' version of something and just sticking it to their car without any investigation, research or testing. It's these guys who give aftermarket modifications a bad name as it's these guys who drive around with incorrect HID installs blinding other drivers etc.
The minute anyone hears you mention that you plan to modify your car people automatically think of big spoilers and day-glo paintwork. But there are genuine 'modders' out there who do it for the proper reasons.
I checked out the HID kit I was buying. Checked it was suitable with my car, it's electrics and most importantly it's headlight set-up. I then asked my local MOT station (friend of a friend) to check that my headlights would be ok after the HID's were installed.
As explained earlier in the thread I then got a mate to check out my headlights for problems such as blinding him as I drove behind him and towards him etc when they were on.
Finally this morning I took my car over to the MOT station and had the lights 'mini-MOT'd' and everything is fine and they 'passed' the MOT.
So now I can safely drive around knowing my modification is fine and safe.
This is the sort of checks and investigations people who want to modify their cars for genuine reason and gain do. It's those who do it to impress their mates in McDonalds car park at 11pm on a Friday night that cause the problems.
If you do it right. Put in the work then everyting will be fine.