One other idea that comes to mind is like Pal-Waukee Airport, in the north Chicago suburbs. I don't think they have a viewing area on-airport but at the south end of the field they have a parking area and set of 'bleachers':wooden tiered seating like you'd find at a sports stadium.
https://binged.it/2EJY3oC
Cheap and effective, the top rows having a view over the fence. Not sure if it'd work in the UK though, someone would mention the 'liability' word.