Re: Xenon headlight upgrade
You can buy H7 Xenon kits with the ballasts, wiring and fittings, but they need to be very carefully fitted.
To the best of my limited knowledge, after a car has been type approved, you can change anything you like about it as long as it will pass an MOT. Only test carried out on lights by MOT testers I've seen is the beam pattern and in one case I saw an MOT tester warn a KEV to take out the 100w dipped beam light bulbs on his car when he got home after passing the test as they were clearly too bright and plod might have a problem. Guess this depends on tester!
Similar argument, additional lights on the front of 4x4s. They wouldn't be type approved as OE kit, just bolted on. Same with bull bars, can't be made with them fitted, but you can then bolt them on when you get home!