I understand your frustration. By the sounds of things, you have already done some testing. It sounds like you have voltage coming out of the fuel pump relay, but not making it back to the fuel pump. This could be due to a failure with the fuel pump cut off switch. This is located at the right hand front door pillar. Sorry no illustration for this one. Here is a wiring schematic of items that are involved in you fuel system. I also would like you to verify that you are testing the fuel pump relay as there are three involved from what I can see.
I must say that I am not a Jaguar tech, but there are some simple wiring items that we need to look at here. Feel free to contact me with any findings or questions! Good luck!