as u_e said, 3,000 people are being admitted to the Filton site, presumably any more would be unmanageable.
It is fairly built up with factories and housing abutting the eastern end of the field so not really much scope for a lot of spectators there. Of course, if the wind is out of the east she will land from the other (Severnside) end anyway.
I believe the best bet would be the Downs. It is a massive open space and the bird will overfly that area, which is next to the Avon Gorge, whichever landing direction is subsequently taken. Local police favour the Downs for spectators.
Local press reported that the Spitfire mentioned is the one owned by Rolls Royce.