Again, it goes back to BA's unique route network and on board product. In the UK, I can only think of Qantas that offers a true four class configuration on their long haul routes, crewed by UK based crew. By its very nature, crew costs are going to be higher at BA because the service can be labour intensive.
As for operating LHR-BOS-LHR as a day return, putting aside the moral and legal issue surrounding such a duty (we'd be looking at around a 16-18 hour duty day, barring delays) while you may save on allowances downroute and hotac, you wouldn't be any better off with the head count factor as crew would reach 900 hours a lot quicker.