From Santorini, I suspect BA will know well in advance how many passengers they have in each cabin. Most passengers are likely to have been away for a set number of days and it’s far less likely than a business route out of Heathrow to have last minute seat changes.

I’d set an alert on EF for when C gets to <2 before I’d consider spending on an upgrade - my suspicion is that the curtain won’t move - but I wouldn’t bet my house on it. But then I’m only 6’3” and I suspect that 6’6” changes the calculation a little.