Voltages look OK - 12.3v is a bit low but about right if the car is open and 'alive'. Looks like the alternator is charging too.
The brake pedal switch can become faulty that can cause the car not to start, as it thinks the pedal isn't being depressed. Are your brake lights working as this may give a clue. Try pressing the pedal really hard when trying to start to see if it changes. I think switches are not that expensive, but swapping out parts at random is not ideal, but you may get lucky. Have you read any fault codes from the car?
I said earlier in the thread that both times the car failed to start the brake pedal was really stiff. I was just searching the forum and came across this:
If you have sat in the car and been playing with the pedal it can pump up the servo to such a level that with normal pressure you can't get enough travel to activate the switch. Simply press really hard on the pedal to overcome this. In future don't sit with the engine off with your foot on the pedal. You aren't the first to do it and I doubt the last. I did this too early on in my ownership of my first XF.
Could that be the problem?