I'm writing this to quickly explain the difference between all of the different types and prices of HID Xenon bulbs - looking down the listings it's a minefield out there.
Why are some £7.99 while others are £100?, well to get a PROPER HID (Hi Intensity Discharge) Xenon bulb to work you need an extra box called a BALLAST, (one balast is needed per bulb, 2 bulbs = 2 ballasts).
As true Xenon bulbs DON'T have a filament inside them they work via a constant spark in a bubble of xenon gas, you can tell a true Xenon bulb by a couple of points, they have a metal rod running up the outside of the glass, they are only 35watt (not 55w like a halogen), and they also usually have the wires connected directly to the bulb.
If you want TRUE HID then your going to have to spend that bit extra and stop looking at the £20 one's, you will want a complete kit - that's ballasts & bulbs, some come with a relay and heavy duty wiring and also make sure you fit an inline fuse (it should come with one) if your going to rewire your car or bike.
I've recently bought a kit from here and I'm very happy with it, the lights are noticably brighter in the reflector and about +10/15% more on the road but it's the colour difference that really makes the difference, as it's so white (4300k kit) it makes reflective surfaces shine MUCH more (road signs, bollards etc) and as I bought this from a safety point of view rather than bling bling extra's for my bike I will be buying another set 'as soon as' to upgrade my car as well to HID.
If you do decided to upgrade, make sure your lights are CORRECTLY aimed at the floor, dazzling lights are dangerous to on coming traffic AND when it's in your mirrors.
I hope this helps - remember the old saying - if it's to cheap then it's probably the wrong thing.
BTW - I don't work for any shop - I'm just telling it like I see it.