Thruxton is a great spectator circuit as it's flat in the middle so you can see most of the circuit. When I go I always stand near campbell I think it is. Good for overtaking under braking etc.
Parking wise it's all a big field. Just get there as early as you can to get closer to the entrance.
In London this weekend, tried to convice the Mrs to go on the way home but not allowed. Rather upsetting