If this flight data in the link below is to be believed, it was light enough to go nearly twice the service ceiling! Or, the scale should read ft rather than m, but even so, that a pretty darn high pop up to look around. I would have thought being at the landing stage after an hour or so flying, the fuel level would be low enough that 2 POB would not be a problem even on a warm UK day. Maybe a lack of fuel the very reason if the big climb sucked more than was bargained for?
https://uk.flightaware.com/live/flight/GCJEK
RIP the pair of them.