When I crank it the rev meter does move

 

In your quest to become a Jaguar XJ12 problem solver

These are your trunk relays up to VIN # X

tem # 3 looks to be the # 2 fuel pump relay to have the correct # installed

Genuine Trunk Compartment Relay For Jaguar Xj6 & Xj12 1987 - 1994 Classic | Jaguar Land Rover Classic Parts

 

When you put a meter on the battery post as it is installed and cranking the engine do you see your 9 volts at that point ?

If you are seeing around 11 or 12 volts during cranking you prove the battery is good and pushing power

if your seeing your 9 volts then the starter system has a excessive drain or the new battery can be bad and should be load tested at the auto parts store

This pushing and pulling notion of what you see as a resulting voltage is how the circuit is working in a dynamic flowing sence and not a still static

you have a couple of terminal post on the battery cable between the battery and the starter solenoid

Obvious observed dirty connections would limit the flow through them so it trades current for voltage and you get voltage drop as the restiction can't satisfy both

In some cases in removing the after the battery along the way terminal post there is a short to car frame ground

Easier then pulling the starter which you may finally have to do

You can remove the B + battery cable at the alternator and place the cable in a leather glove