I have NEVER heard of the RAF charging for a simple visit/interview facility (that doesn't mean it doesn't happen, of course), though they are increasingly charging for flying (even in a tanker, I believe).
I don't know who has commissioned you to write this series of articles, but I can't imagine the RAF charging any of the mainstream UK aviation publications, so I wonder if the charges are being threatened as a deterrent because they don't like the title, the editor or ....... you!
Now I've only been working with the RAF as a journo on and off for 23 years, but if you want to chat about it with a newbie, Ewan, PM me your phone number if you don't already have mine.....