Really struggling with this one, and need to go back to the very beginning of symptoms as got distracted with fuel issues and it's not that.
2003 Disco 11 TD5. Been a bit iffy starting in the mornings for a while. Little white smoke on start, but cleared on driving. Once started and going went well. If it was turned off and re started within a couple of hours or so no problem. But if left all day at work would struggle again. This got progressively worse, with more smoke. Checked oil and coolant... No contamination. Coolant wasn't really going down. Temp stayed constant. This is when we started to look at fuel issues. Had lots of spares here from a mate who had sold his and so luckily didn't cost us much! We had the injectors all checked professionally All fine. New seals and loom. Seemed a bit better for a couple of days. Sometimes would start straight away. Other times it really struggled. Then it didn't start at all. Loads of white smoke. New fuel pump. New fuel filter housing. . New starter motor and CPS. Opened oil filler cap and was breathing while cranking, which has now led us to believe head gasket. Taking it to bits today to go and get head pressure tested too. Oil in the intercooler pipe. Lots of oil. Plus oil on cylinder1 of the glow plugs when we removed it. Pressure tested it here and ranges from 100psi on cylinder 1, 200 psi cylinders 2/3 and 260psi cylinder 4. Nothing completely obvious on the head gasket, but Am I being too hopeful it is 'just' a head gasket or are we looking at worse...

 

Air coming out of the oil filler cap might just be a sign of air going past the rings. This is shown by your low compression on cylinder one. It is possible the ring sealing issues go away when the car is warmed up and running. If you do not have overheating, coolant loss, or contamination, then you probably don't have head gasket issues. Are the glow plugs and glow plug system in good condition? I have a diesel truck with a few bad glow plugs that is hard to start and smokes on startup.

 

A bore scope if you can fit it might reveal the shape of the the cylinder walls and visible piston surface areas.

I'd expect diesel compression, warm, with all glow plugs removed, fuel to the injectors cut off or injectors disabled and anti shutter/runaway valve open (if equipped) to be 300-400 psi. I also want them to be similar between each cylinder.

A weak starter or battery or failure to allow air into intake or failure to remove all glow plugs will yield much lower and inaccurate results on a diesel.

If the head gasket is the expected culprit Id probably spend an afternoon pulling the head to examine assuming those compression numbers are correct.

At that point the head gasket, cylinder walls, piston height/condition and head can all be inspected.