I agree with NWSRG. Basically, BAA view spectators as a nuisance and distraction from making ever greater profits and as far as I know, there are now no viewing facilities at any of their airports. As you say, the QB terrace was closed many years ago as part of a drive by BAA to get rid of spectators at LHR. I spent a few days there in my planespotting time in the first half of the 1970s-happy memories.
OK, BAA have a business to run, but is it really asking too much for them to provide a facility say half as good as the excellent viewing park at M/C (bar the exorbitant parking prices there now)?