You can use the Concorde Room if you are flying F on BA not QF. The F lounge downstairs serves substantially the same food and drink now as the Concorde Room (most of what made the Concorde Room special was retired along with the Concordes and now it is only marginally nicer than the F lounge; a sad fate for a great room). You probably won't be able to get a treatment at the MB spa with such little notice, the bookings fill up early in the morning and you won't have any priority. As for watching aircraft, there is a reasonable view from all of the BA lounges at LHR -- take your pick what you want to watch.