Just to clarify - as Dink says H4 bulbs use one bulb to provide the high and low beam and this is the reason you see H4 Bi-xenon kits etc as you get some H4 kits that only give HIDs for low beam and some (Bi-Xenon) that give HIDs for high and low. THis is only used on Mk1 Clios and some very early Mk2s, all other Clios use seperate bulbs for high and low beam meaning that you will only get HID low beams if you buy a kit, and you will retain your halogen high beam.
In terms of legality, being totally honest the vast, vast majority of HID kits fitted are illegal on the car. The kits themselves are not illegal, but there are a number of things your car, and not the kit, need to meet in order to be legal. This includes things like headlight washers which most people don't have. However, that is the individuals choice whether to risk this or not and I haven't been aware of any of my customers having any trouble with the law about their lights.
In terms of passing your MOT there is NO REASON that HIDs will fail your MOT as long as you have your bulb alignment correct - so it is exactly the same case as with halogen bulbs. I know this after speaking to various MOT testers and they have no reason to fail HIDs as it is simply not something they check as part of an MOT.
If anyone wants more info or wants to buy a kit please PM me. I have been slow recently getting back to people however I still have kits available and those who have bought from me previously will back up that these are excellent quality kits at a VERY reasonable price. In addition to this I will always try and help resolve and fitment problems people have and if you require lights for a clio I can advise exactly what kit is correct for you whatever shape/style clio you have. I can also supply kits for other cars, just let me know what fitment you need.