To some extent I agree (especially if you could get a Wimpy airborne for less than a million!).
But I suspect Brooklands are being a bit canny, knowing that not only will the Conc be good for them in terms of visitor numbers and publicity, but more importantly the building to house it will bring a few other airframes indoors anyway. Somehow I doubt an appeal for a million quid to get any of their current outdoors collection under cover would be very glamorous and therefore get anywhere - but a pile of cash for a Concorde with all those Concorde fans willing to contribute and/or get involved with the restoration of the only one that will be publicly accessible anywhere near Heathrow... much easier.
And as always with appeals for huge amounts of moolah I bet they really only need about xx% of the figure and anything else is a bonus!