Depends on the type of car
See my post above, about Pug TD HG failure
Running aside, if it was (over) pressurising the coolant, there would be bubbles in coolant. Lots of bubbles.
As stated, the engine seems perfect until you start putting under load...
This is going to be an interesting one, whatever the outcome
I agree with everything said, about it's unlikely HG... but the one thing I've learned, is never take anything for granted on Omega's

You must be feeling a bit down about it... I know I did, when my old 2.0GLS project refused to start whatever I did.. you start to loose all faith in the car..
But please don't - you will get there.
If at first you don't suceed...
This is only a thought... but I wonder if someone put radweld in it at some point to try and cure a small leak?
I once, (long time ago, I've learned a lot since!) stupidly put Radweld in an Omega V6... in short, it ruined the engine.. constantly over-pressuring, running too hot, blowing from the cap etc... I can only imagine it clogged up the tiny waterways..