First up I do electrical and electronic diagnostics for a living, also own a couple of Jags
I had a customers RHD 1996 3.2 Auto XJ6 to work on with the problem in the title that I found the solution too, I think it may be a common issue but judging by the number of unresolved threads maybe not commonly found.

This applies only if you have the following :
No Crank. Check engine light comes on, but goes out after a few seconds and does not stay on until you crank - this means you have an immobiliser problem.
Probably a battery drain as well.
Ideally you should have tried more than one key, and used an ignition detector ring to prove no key exciter signal.
Basic multi-meter skills

In summary the problem is a short to ground of any of the items on the Ign pos 1 Ground switched circuit like this:

This is because the SLCM expects to see no ground on that circuit (17 I) then ground as the key is switched on, I think it uses that change of state to send a serial encoded message to the key exciter to turn on and read the key. - Whatever, it does not work if that change of state is not there and the key reader is not actuated.


Easiest way to check this is to remove the lower column cover and follow the wire from the back of the ign switch to a nearby white block FC54 as per top diagram.
Measure voltage from B+ to the WN - White and Brown wire pos 2.
Key off there should be no ground so 0v, key in pos I (ie ACC) you should have 12v/B+ , if you have 12v/B+ with the key off/out then this is your problem.
If you split that connector you can see if the ground is downstream or in the ign switch. If you then go to behind the glove box by the end panel you will find a big black 48way panel connector FC5 underlined above, split here and narrow it down further.

I was lucky having done the above I found a short to ground in one of the interior lights on (13 I) , but any of those items using a ACC switched ground with a short will stop the car from starting.

In this case the interior light immobilised the car.

Hope that helps someone, if you have a different set of circumstances please start a new thread and I am sure someone will help you.
If this is your issue and fixes it please make a note here .

If any of our knowledgable members know if other Jags have the same immo set up please post so the search finds it!

 

You could be on to something.

When exiting the vehicle and removing the key it makes a linier switch in the ignition switch on pin4 not shown in your pic

This pin 4 is wired directly to the SLCM

This starts the process on security lock control module to confirm things before it knows the car is secure and goes to sleep

On entering the car and placing the key back in without rotating it it will make the linier switch again , the question does this wake up the SLCM

Or does the rotation to the 1st ACC position wake up the security lock control module and take the immobilization away?

 

I checked the Key in switch at the BCM as it goes to both, and it was working.
There was no serial comms from the SLCM to the immo unit to tell it to turn on and read the key at that point. Once the extra ground was removed by unplugging the light it was getting the wake up signal from the SLCM and reading the key.
Definitely not working without seeing that state change on the Pos 1 ground.

 

In the all encompassing theory of X300 operation from Parker

In the broad picture of the steering coluum and seat movement vs. a full lockup up like the doors and trunk

The security lock control module exiting commands may split into 2 sections , driver / passenger convenience and full lock up

Your interior lights I think would be a convenence item

Let me put a couple of prints together and see how they work together.

My computer has issues so I will be posting the work as progressed.

We do not have the chip in key exciter / transponder / decoder in the US but I did see it woven in on a print somewhere

Your pin 2 of the ignition switch brings about a whole understanding of what can fail on the Immobilization or lack of as the term can be miss aplied in what it is actually doing , enhibit / enable , signal or lack of signal and even a enhibit / enable serial command.



A quick remedy of your current issue is I found a " Hard Reset " works for me but we do not have the stereo head security lock out / up and I'm not aware of how you work around that in the UK models

From reading others a common problem is the linier key in key out switch.on ingnition switch pin4 to 5 ( car frame ground } can stick

And the steering column can be binding to not get to the retract position

This can prevent the SLCM logic steps from getting to zero state. and your observances may not be after the SLCM gets to zero state

The SLCM is hard wired to the battery on fuse # 3 or $ in the trunk for it's main power source and the longer the hard reset shorting like overnight the better

The main purpose of the ignition wsitch is to provide a ground path which is what you my be seeing in your observations not at the correct key position

The ground path would remove both a feeler voltage and serial command.

Both kinds of electrical signals can exist on 1 wire at the same time

The ignition swiitch.does provide a ground path for the king relays on your fuse boxes as they in effect are putting out a feeler voltage on the control half of the relays seeking a ground to close

What I found on mine was a corrode pin 5

This pin 5 to car frame ground stud can get disturbed and degraded from 0 resistance.which is what you want

Editing

Note that the Decoder ( what is the decoder ) is not wired into the SLCM but the BPM for starter enable

The Check Engine Light can be added inside the BPM to the wire going to the light by way of the instrument cluster

For the Check Engine Light to be active .............researching












Upon entering the car a door switch should turn on the interior lights

Before ignition key insertion pin 1 should not see a ground

After key insertion pin 4 will make

on first position key rotation pin 1 will see a ground and close the king relay in the corner of the trunk fuse box for stereo power

On 2nd key rotation will close the rest of the king relays to have the car fully powered


" In summary the problem is a short to ground of any of the items on the Ign pos 1 Ground switched circuit like this "

Working as designed with key rotation position 1 providing a ground path through the ignition switch

" This is because the SLCM expects to see no ground on that circuit (17 I) then ground as the key is switched on, I think it uses that change of state to send a serial encoded message to the key exciter to turn on and read the key. - Whatever, it does not work if that change of state is not there and the key reader is not actuated. "

The ground should not be there with your keys in pocket and possibly putting things out of sequence.

" Key off there should be no ground so 0v, key in pos I (ie ACC) you should have 12v/B+ , if you have 1" 2v/B+ with the key off/out then this is your problem."

This is opposite of print derivation , key off you should see ground seeking feeler voltage.

You may have a intermittent issue with the large BT4 connector above the fuel tank:, this connector can be trouble connecting ( sheet metal cutout in the way ) and there is a tie wrap on the connector connection lock or like on the cabin dash behind the glove box FC5 connector

 

^ Thanks for the extensive reply (and I love the annotations!), however I solved the issue in Post 1 and was posting about the solution to help others, I put that in the title but realise it is perhaps not clear.

There was a hard, permanent ground on the Pos 1 key circuit that should have been switched. This was being provided via (what I thought) was a faulty rear light.
I proved repeatedly that if the SLCM does not see that change of state from no ground key out/pos 0 to ground key in pos 1 on that line (CA18-8) it will not activate the immobiliser/decoder (via serial message) module to read the key.

Sorry should have explained I had the meter and a test light rigged from B+ to the Ign pos 1 circuit with the key out I had 12v (showing the ground fault) I was looking to get rid of that ground. Unplugged a rear light that was broken and the ground went. so the meter showed 0v.

HOWEVER - turns out I was initially wrong about the root cause of the issue, which was in fact a hard ground on the interior lighting circuit ( F#8 RH Heelboard) this meant that all of these red boxes were a ground not B+






including the 2 rear E-post lamps that then have the crossover to the Ign Pos 1 circuit - the lighting circuit fault was backfeeding through the bulbs. I eventually found the lighting ground circuit in the pass sunvisor wires chafed between connector and visor.

So a light did not kill the car - a sunvisor did. Fixed the chafed wire and no more ground on the the lighting circuit, no more ground on the ign pos 1 ground circuit, car starts no issues.

So for avoidance of doubt this is a solution, solved, customer happy, not looking for help, but posting for others information