Several thoughts:

Please give us a quick rundown on recent vehicle history. You've had several threads going and I can't remember exactly what has been done. I vaguely remember new head gaskets and a new alternator, but am not positive.

If the alternator is new, most aftermarket brands won't communicate properly with the car. It will charge just fine, but the little red battery symbol stays illuminated.

Please make sure the battery is fully charged before doing anything else. This is VERY important. Your description sounds like a dead alternator and a partially discharged battery. When voltage drops low with the engine running, most non-essential functions (gauges, for example) are shut down to save energy for ignition and fuel delivery.

Check the alternator output. On an early model, you should see about 13.5v at the battery.

 

I had some similar weird stuff going on with my 2.5 V6 last year.
It was a poor battery connection on the earth lead.
Previous owner had fitted a battery clamp that looked like it came off a tuc tuc.
Looked ok connection until I loosened it and took it off to clean it then the clamp fell apart.
Got a proper battery clamp now with quick disconnection without removing the battery clamp.

 

Another thing, check the boot/trunk for water ingress.
Not uncommon on the S type to have water splashing around which if it gets deep enough can wreck the electrics in the boot.

 

 

You have to get the electrical issues sorted in order to form a baseline for further troubleshooting. If the alternator just produces "some sort of current" and the battery and cables are marginal, the car is highly unlikely to run properly and spurious codes, together with electrical gremlins, will continue to the the order of the day...

 

 

18 volts is too much on a 3.0; the charging voltage should be 15-15.5 volts. Also what is your static charge at the battery after loosing the surface charge by leaving the headlights on for three minutes? Improper pre-start voltage can cause all sorts of gremlins...

 

do you have a way to read jaguar specific codes....the car electronics protect themselves for an over or under charge....18 volts is definitely over as S-Type Owner stated....an over voltage will set a code in a few modules I know because it happened to me when my alternator went bad...luckily I was in the driveway, and the same thing happened...all sorts of strange lights and the car shut off...I ended up being able to re start and got 18.5 volts reading and replaced the alternator with a used oem one.....there are a few different alternators that will fit these cars, so you have to get exactly the same jaguar part as the old one.....especially if it has the "smart charge" system

 

At 18V don't be surprised if some modules are damaged.