Well, if it was just for the slots, BA could have an operator doing a daily EMA-LHR run with a Shed instead of flying a 747 across the pond. I seem to remember that Virgin exactly did that at LGW and used a small operator to keep its slots warm by flying EXT-LGW.
So apparently these ghost flights are indeed more some kind of postioning flight as the return flights can be crewed as planned in the US.