xj8 boot wont open

You put the key into the hole in the logo on the right hand side below the light cluster and it would not turn?

I don't have an XK8, but I have an XJS and I know there are probably several possibilities that will be covered in your owners guide if the XK8 has a similar setup to my XJS...

1. there is a valet key that will not open the boot, you must use a regular key
2. the keylock position (set with a regular key) determines whether the trunk opens and under what conditions (clockwise stop, vertical stop, counter-clockwise stop each set different conditions electrically)
3. if you can open the hood (bonnet) and find the underhood jumper connections (termination points of the battery cables that run from the boot/trunk to the engine compartment) you could use jumper cables to power the car with your battery and perhaps then the boot/trunk lid will open normally.

You can also use one of those battery jumpers that plug into the cigarette lighter socket. That would give you power to open the trunk/boot.

Paul