In short you can use a scanner to listen to anything broadcast for general reception, radio amateurs, CB, weather and navigation broadcasts. Unless you are a police officer or work in the emergency services you are not allowed to listen to their communications.
You can only listen to other services if you have the permission of the sender. The air show is a good example where the control tower frequencies are publicised and that would be considered permission.
From the other thread referred to above. This is Ofcom's interpretation of the law. Given that ATC frequencies are published, I can't see where the illegality lies.