This video shows how to replace the HID headlamps on a Jaguar XK8. I'm not sure if this also applies to the XK8s with the regular, halogen lamps. It might. Hopefully someone will let me know.
By the way... you can buy a pair of replacement bulbs on eBay for about $32 or so. Make sure you order D1S bulbs, and look at the picture of the bulb to make sure it has the big square igniter on the back of the bulb. There are retrofit kits labeled D1S, but the retrofit bulbs have the igniter built into the ballast, and they won't work in the cars with factory HIDs.
Jaguar, XK8, XKR, X100, HID, headlight, headlamp, replacement, how, to, How-to, bulb