KLM have operated a morning 763 to/from Heathrow for years, mainly for the benefit of the heavy North American connection traffic.
Not entirely correct. The B767-300 is there for the benefit of the Accra flight. If the 767 does the AMS
they only have to get the containers out of the aircaft onto the LHR
flight (if they have to unload them at all because they regulary use the aircraft inbound from ACC for the outbound LHR
flight). They do this not only with the usual LD2 containers but also with LD3 containers (if the ACC flight is operated with a MD
-11).. These containers are filled with Accra bound pax at LHR
in order to have an effecient transfer. They used to do this also for the Lagos flight but since the Lagos departure ex AMS
was moved earlier they couldn't do this anymore.
The second reason is cargo, as the 767 can transport pallets and LD2 and LD3 containers which the 737s can't.
Finally, the first two flights flights to LHR
is usually packed with business traffic (although most business traffic on the first) and the second departure can catch transfer pax.