Originally Posted by jsloan

There's no aircraft in UA's fleet with twice the capacity of a 764.

Furthermore, this is the offseason for NYC-LON travel. However, slots at LHR are precious, and if UA were to try to operate fewer flights during the offseason, they'd lose the slot -- permanently. They're essentially required to run the flights, and they're limited as to when they can run them because slots are assigned with a given time window. So, you end up with winter schedules that are designed to cause as little overall harm as possible so that they retain the ability to make money during the profitable summer schedule.