A 4 cycle Honda cycle? I don't think the amperage will be enough from
a  bike.  You probably need to let the battery come up a bit, so jump
it with another auto and first  leave the donor car running for a
while. Make sure the connections are good.  

The size of the engine makes no difference -- it is the size of the
alternator and the electrical power than is the concern.  Usually a smaller
car has smaller amperage needs.  Try it again with the Honda but rev the
engine in order to take the RPMs of the engine up. This should produce
enough electrical power to start the Jag.

Make sure to put red to positive and black to negative off the Honda battery
and Black to metal on the Jaguar so you don't fry anything on the Jag.

How long did your XK8 sit?   My XJ40 sits for a month at a time and starts
right up.  You may have other problems with your XK8.

Webserve.

Is the battery completely flat ?  If so you are probably better off
disconnecting and recharging the battery prior to re-starting and resetting
codes.  You are advised against jump starting modern cars due to potential
damage that can occur with the ECU.