Problem is whilst there might be a basis in fact ( usually starting with somebody running a feasibility) I think those rumours can become circular...the extra 380 rumour also has been running eslewhere, not just on pprune or flyertalk but there's still nothing concrete. On an in house forum it started last week but in that varient it was triggered by someone claiming he had heard a story from a senior cabin crew member who had served Mr XXX who had told him that , etc etc...
As for specifically the 777W rumour, all I can say is that in all honesty that one wasn't circulating even amongst the BA 777/77W community in the last 24 hours so forgive me if I for one take it with a pinch of salt, at least for now...
That said over the years rumours have always been that BA would take more 777Ws if they could get them, much more so than 380's.... so if you want to go with a rumour I'd go with the 77W one.
( Tin hat on, retires to safe distance........................, and add's rider that with it being BA anything could happen tomorrow..))
Edit to add: ETOPS that belatedly rings a bell and would make a bit more sense - unlike fleet expansion I doubt a one for one (or three for three swap) would raise much comment if/when that happened..