Try every key.

Trying to get into the trunk using the mechanical key release in the "XK8" never worked for me. I figured the mechanism was stuck. But I just tried my second black key and it opened right up. Perhaps the wear of the key used in the ignition affects the fit?

Hoping this helps someone else save some effort.

 

I believe it may be because one is a valet key that is unable to open the trunk

 

You can always hook up a battery/charger to the under hood connections and open it as usual.

 

Green key is the Valet keys.

 

If the battery is dead, the doors won't open with the key, so it's hard to open the hood. If you can get into the trunk, you can power the system that will open the doors through the battery connections.

Yes, the green is the valet, which is why I mentioned it was my second black key that worked.