Interesting conversation. While my airline industry experience is more US based. Some of the US carriers also do maintenance at outstations where it is cheaper - not sure about BA. Also keep in mind that the way a plane rotates through the network, one low utilization day may be followed by nearly 24 hours of flying. A US hypothetical example: one day a UA 777 might fly LAX-IAD-LAX (say ~11 hours of actual flying in a 24 hour period), the next it could fly LAX-NRT-SIN (say ~18 hours of flying in about ~24hours). Remember just because BA can return the 747 to LHR earlier vs. having it sit on the ground, it might not have anywhere else to go. Based on time zones and flight times BA might not have anywhere to send a 747 so you might as well have it sit at XXX and return the flight at a time when yields might be better. Also keep in mind crew rest period dictate some of the flight planning. Depending on contractual/legal requirements, having the plane return earlier might create the need for another crew on layover in the city.