Back at NH
According to previous posters, the prime pump switches were probably turned on at the conclusion of the final tasking some 20 or so minutes prior to arriving back at Glasgow controlled airspace.
To the best of my knowledge, there is no reason for the prime pumps to be selected on during normal flight. They are only used during engine start and in flight possibly to diagnose abnormal fuel supply problems to an engine.
A prime pump could be used during an attempt to restart an engine in flight although I seem to recall that engine restarts in flight are contrary to advice in the RFM. (I might be wrong on that point).