There is no way you would collect a large load of pax at LHR, fly up to MAN with them on board to pick up a few more and then continue as planned if it was a normal scheduled flight.
This would also not have been traffic rights related as 1 flight in a blue moon means nothing, as I understand it.
I believe it was a religious group who had a party large enough to fill the aircraft, so it made sense to take the flight from MAN.