For some reason most people assume that (a) nobody flies very far outside the UK, and (b) the pilot had no fuel left.
I am looking for more general references (ICAO, perhaps, but one could start with say Italy or Greece) on whether or why the pilot+passengers have to enter the terminal and clear Customs when it's just a fuel stop.
Airlines and bizjets do fuel stops routinely, without clearing Customs. I did one a few years ago (747), with everybody remaining aboard. How does that work? In GA terms, I have refuelled at say Biarritz on a UK-Spain flight without going anywhere near Customs (stood around the aircraft).
One obvious issue is when one needs to leave airside to pay for the fuel.
mm_flynn - you assume correctly and that (PPR for Customs, or part-time Customs) is probably the heart of the matter.
However, take this example:
Pilot 1 lands at 0900, having given PPR for Customs. He is OK. "Airside" is "airside".
Pilot 2 lands at 0905, not having given PPR for Customs. "Airside" is not "airside" now??