1. install a battery disconnect switch in the engine compartment next to the battery.
napa auto parts sells one for under $ 20.00
2. look for battery drain.
a. disconnect battery, check battery voltage after 1 day, if voltage is 2-3 volts less than full charge, battery is faulty.
b. checking for a drain ( parasitic leak ) involves disconnecting circuit by circuit to locate problem area.
c. one thing i have noticed is that a very large percentage of jaguar xk8 in your model year suffer from this malady,,apparently the electrics are not as reliable or as high quality as american, german, or japanese cars.