I would ask the shop for the part number and brand of the pump they plan to use to independently verify it is suitable for the XJS V12. It does seem the same pump is commonly advertised for use on both the XJS and XJ6 for 1992.

If they haven't pulled the tank out yet, I don't see how they could know if the evaporative flange, wiring harness and sealing ring would need replacing. It's common to replace a bad pump and re-use everything else except perhaps the sealing ring. It's not a small job to remove the tank so I understand they may "recommend" replacing those parts while they are in there but I'd consider reusing anything you can.

The electrcal connector in the flange seems to be a common failure point but a cheaper solution than replacing (as discussed further up in this thread and in Kirby's book) is to run wiring directly through the the flange or run threaded posts through it with wiring connections on top and bottom of the flange.

Did you or did the shop try putting 12volts directly to the pump harness to see if the pump would run? Sometimes the fuel pump relay is the problem. They may not want to do the job without replacing the parts they recommend replacing, or they may tell you there will be no warranty on the work if not replaced.

You may want to post in your geographic section on these forums to ask for Input on this shop or another shop in your area. Or seek more input in the XJ40 section as they are also familiar with these pump/flange issues.