Where did the whole LHR-BOS-LHR day trip idea come from? Thats never going to happen, you couldn't get close under FTLs. BA's crew costs are not high because of the complexity of service, they're high because of the reasons in the Columbus document:
Crew on legacy pay scales thar are double the industry average.
4 supervisory grades out of 15 crew on a 747.
Rigid inflexibility and sky high costs when off schedule.
Having 4 cabins doesn't make the crew cost more, the complexity only means it takes longer to do the service.