Some of the Jaguars has a place to insert the key above the license plate to the right hand side. If all else fails and you can open the hood, at the fire wall there is a stud on the passenger side with a black nylon or rubber cap over it, it has a heavy gauge cable attached to it, you can carefully attach a jumper cable positive to that stud and the ground from our jumper cable to any ground source on the engine or body. If you can't locate the power stud the starter (underneath the car) has a large lug on it with a heavy cable attached that runs back to the battery positive carefully attach the power and ground from a jumper battery to that cable. Be careful not to short or arc the positive cable to ground.
You are a genius. I love you. I disconnected my battery and closed the boot. Not thinking that I only had the Valet key that will not unlock the boot. After standing in 19 degree weather with 6 inches of snow on the ground for 4 hours changing the valve cover gasket (and Mr. Murphy came to visit often) I realized I could not get into the boot. I have not tried jumping the cable but I knew there had to be another way to get into the boot in this situation. I am now warming up with a Russian drink of choice (only because I am out of Guinness and Jamison) but I will try your trick in the morning. Thank you so much for sharing your wisdom.