Maximizing the utilization of big jets on dedicated international runs is very difficult for a variety of factors, and thus layovers are often calculated into the economics of these routes. Given the great length of time such aircraft are in the air getting from LHR to their various destinations, time zone changes and arrival times, which often dictate night departures from the UK and morning arrivals at many destinations — just after the lifting of curfews — makes immediate turnarounds, while convenient for the airline, difficult. And in some cases, would actually result in needing an extra aircraft to provide appropriate service on a route.

For example, southern African flights leave LHR in the evening and arrive at their various destinations in the morning. Returning these same planes to LHR within hours of their arrival would mean they would be arriving back close to the evening curfew. Too late for the same aircraft to be used on the evening departure back down. Spending a full day on the ground, while not ideal in terms of utilization of each aircraft, does permit the use of two rather than three aircraft on that route. An obvious savings which would off-set lower inflight hours. (And since many of these routes rely on business travellers, the idea of losing a full day sitting on an airplane could affect choice of carrier. Far better to fly overnight in each direction.) Of course, lower inflight hours also lengthens the lifespan of an aircraft.

I believe BA has the same issue with its flights to Australia and Hong Kong, which generally have daytime layovers, arriving early morning, but not departing until late afternoon or mid-evening.