"And is there really nothing they can do with the aircraft while it is in Sweden for five or so hours?"
Ok 5 hours - allow 45 minutes to board new passengers - then fly somewhere - that's an hour otherwise it's not competitive with surface transport - then an hour on the ground and an hour back and deplane 30 minutes - total = 4-15
and the risk you delay the ASTRA ZENECA departure ... and they are the ones paying you serious money (far more than you'd get for a normal UK- Scandi flight I'm sure - that £1300 return probably isn't too far off the cost per pax)
Just not worth it -a very small increase in income for more miles on the 'plane and the chance of p****** off your big customer