They ought to put the Manchester guys in charge - they have realised (as happened for the Champions League Final) that a huge number of extra spectators will be around, and have planned to accomodate them with extra capacity at the Viewing Park. They are not a charity, so I imagine they made sufficient profit on the Champions League effort to demonstrate that doing this is a commerically sensible move, as well as getting all the spectators in a controlled area, out of the way of regular passengers.
This particular case is not a spotters issue, as Concorde generates such interest in the public generally that I suspect that spotters will be a minority at LHR on the 24th. These days a sensible aviation company would leap at the opportunity for good publicity....
Given that BAA are normally not backwards in trying to make a few quid from airport visitors I'm amazed that they haven't tendered for a 3rd party to run the viewing facilities on a regular basis - Frankfurt do it in a secure manner, charge a reasonable sum, make a profit from it, and are one of the biggest tourist attractions in Germany. Anyone with any funds to put together a business proposition? I would do it, but I'm taking on a mortgage today to that makes my eyes water!!