Seems to me that there are some points being missed here. Clearly there is a 'drop in' clientele for the cafe and a need to control entry to the wider site. I can't see any reason why both cannot be done without the need to charge £15 for someone who just wants a coffee and chill.
It wouldn't take much (surely?) to control entry to the hangars and avoid alienating what would appear to be a significant part of your faithful customers who only want a cup of tea. It's a bit smug to talk about 'how much it cost to get there' when OAPs and others have to watch every penny.
So yes, a £30 membership makes sense, but that's quite a few cups of tea.