S80: Engine Odd Coolant Issue

[Captain Fuzzybeard]

What follows is the oddest story.
We bought a 2013 plate S80 I believe it is the d3 2.0 engine. and took it home to Devon from Scotland. Anyways parked overnight at Stafford. In the morning coolant level low.
Undid cap. Coolant rose to correct level and so did back up and drove to Devon. Got 8miles from where we where staying coolant level low. After a good few hours of driving.
Anyway to cut a very long story short at this point things went very wrong and we had the following done nd replaced.
K seal (I know it didn't fix it just made things worse)
Replaced
Coolant expansion tank, radiator, intercooler. Permanently pulled thermostat.
Passed compression and blue dye tests.
Still no dice.
After limping the car back home and only taking it on short trips my bank balance refreshed. Local garage here did a compression test again all good. But we will replace the coolant expansion cap and that should do it. I picked it up Friday night.
Saturday -test drive. 150 miles to Inverness not a problem.
Eat lunch. Turn around drive back. About 10 miles into return journey. I turn off the fan in the cabin. (Which has been on and hot all day.
10miles later coolant level low. I unscrew the top it bubbles back up. 15miles later coolant level low.
I assess with my wife what changed. We turned the heater off. So we turn it back on and for the rest of the trip. Nearly 100miles it is absolutely fine.

So why? Is there a heater bypass hose or something that has a hole in it causing pressure to build up?

I must be the only person in the country that doesn't want to bypass my heater core.
I want to bypass the bypass or whatever section it is that it goes down when the heater in the cabin is not in use.

Can anyone help?