This is just my line of thought from doing research on this issue today. Take it with the obligatory grain of salt.
Quick relief - perhaps you can use a funnel to fill up your car. This way, pressure can escape from the fill neck while you are filling, but it won't cause the gas handle to shut off. Messy, but might keep you on the road if this issue becomes chronic.
The issue is - the gas pump shuts off while adding fuel. So, why do gas pumps shut off? I have read three websites this morning that claim it is because gas covers the end of the fuel nozzle, here are two:
The Straight Dope: How does a gas pump know to shut itself off?
HowStuffWorks "How does a gas pump know when my tank is full?"
This means that gas is not properly flowing away from the gas pump handle while you are filling.
Hypothesis #1: Gas is filling up your filler neck because of an obstruction in the neck.
There is a check valve at the bottom of the filler neck that is supposed to prevent gas from surging back at the operator:
2000 Jaguar Xj8 Problem Fuel Fill
The check valve in the tank sits in the filler neck, but JTIS says the filler pipe is integral to the tank and non adjustable, so that sounds like major work.
Also can't find much about this on the net, so, I am bypassing this as a hypothesis right now.
Hypothesis #2: Pressure in the tank is preventing gas from flowing away from the gas filler neck.
I imagine if the tank was not properly vented, and the fuel was being vented back up the filler neck during filling, gas would not flow away from the neck properly, causing the gas pump handle to click off.
So, how do these cars vent the tank when gas is being added? It looks like your car has the On-board Fuel Vapor Recovery system introduced in 1999.
Standard Features - 1999 Jaguar XK8 Convertible - Yahoo! Autos
Found this document about it on jagforums - the issuing website is out of service, but a quick DNS lookup shows it used to belong to JaguarLandrover,53S/12/2, Lode Lane, Solihull, West Midlands, B92 8NW, UK:
"The fuel filler pipe has a reduced diameter between the nozzle guide and the tank, providing a liquid seal when refueling and preventing the fuel vapor venting directly to atmosphere.
There is no breather tube fitted between the tank and the filler nozzle. To prevent spit back when refueling, a check valve is fitted at the lower end of the filler pipe inside the tank.
During refueling, the tank is vented via the fuel level vent valve, large bore vapor pipes and the charcoal canisters. The fuel level vent valve incorporates a float valve which is closed by the rising fuel level, creating a back pressure and causing the fuel delivery to stop.
In the closed position, the fuel level vent valve also sets the fuel level. With the fuel level vent valve closed (tank full), any increase in pressure or overfilling is relieved by a separate rollover protected grade vent valve.
The outlet from this valve feeds into the main fuel level vent valve vapor outlet pipe, by-passing the closed fuel level vent valve.
When the fuel level is below full, the fuel level vent valve opens to allow unrestricted venting via the canisters. A pressure relief valve is incorporated into the fuel level vent valve assembly and has an outlet pipe to the filler nozzle.
If a blockage or other restriction (eg, evaporative emission canister vent solenoid in the closed position) occurs in the vapor vent system, the pressure relief valve opens to allow venting to atmosphere via the filler nozzle guide and fuel filler cap."
Looks like pressure is vented first by the vent valve to the canisters, and if the tank is full, then by the grade valve pushing out through the vent valve tubing, and in case of blockage, the pressure relieve valve built into the vent valve will push tank pressure back out the filler neck.
The vent valve has an associated solenoid that can close and block the ability of the valve to vent, but JTIS says this is just used by the computer (in conjunction with the tank pressure sensor next to these vent valves on the top of the tank) during testing, so it should be normally open.
This means that if your vent valve doesn't open for whatever reason (failed, or failed solenoid), and the grade valve can't vent pressure through the vent valve tubing, then the pressure will be pushed right back up the filler neck by the relief valve, which (in theory) would push the gas back against the filler handle, and cause the gas to shut off.
So - I would chase these things - vent valve solenoid, canisters, vent valve (and integral pressure valve), grade valve.
Personally - I am suspecting the vent valve solenoid or canisters, or associated piping. These items are common to the vent and grade valves. It seems more unlikely that both vent and grade valve will have failed.
First - check for codes. I read that you did this earlier, but emissions codes will prevent the ECU from purging the canisters.
Then, disconnect the line that connects the vent valve to the vent valve solenoid. When you fill up the tank using the funnel, does pressure come out of the vent hose, or does it come out of the filler neck?
If pressure comes out of the hose, try leaving the hose unattached and filling normally. Of course, at this point, you will be venting raw gasoline vapor around you and your car while you are fueling, but you will know it was a blockage in the vent line - solenoid, kink, bad canister, something like that.
If the pressure is coming out of the filler neck, even with the vent line open, then the relief valve is shunting pressure, so why is it doing that? JTIS says the canister vent solenoid is attached to the canisters, so I would test or replace that as a first step.
If replacing the solenoid does not fix it, then either the vent valve AND the grade valve are failing, or the canister is not taking pressure.
Can you detach the solenoid from the canister and fill the car? Does the solenoid vent pressure when you do this? If so, then the canister is broken. If not, then your vent and grade valves are kaput.
For some interesting reading on older Jag issues with this, go here:
If you need it, this link shows how to remove the fuel tank, after making a special tool to do so:
I think the "Canister Control Valve" referenced in this thread is called the "Canister Purge Valve" by JTIS. Purging is the computer's way of taking vapors out of the canister and burning them in the engine.
Both JTIS and that PDF seem to show both vent valve and grade valve in the fuel vapor vent valve housing, right on top of the gas tank.