Cosford of course is RAF Museum, not IWM but they're all just as bad. I'm sure Hendon and Duxford have held auctions - classic cars? - in the past and I bet Joe Public isn't allowed in those areas. I suppose Hendon could say admission's free so what are you complaining about but Duxford costs a few bob to get in.
We had a similar experience on the SS Great Britain 8 or 9 years ago when a big chunk of it had been hired out for a wedding reception. Great venue and I hope they enjoyed it but what about Joe Public, which in that case included me, who'd paid for access.
I guess the excuse would be it generates valuable income, and I'm sure it does, but you can't just ignore the general paying public, especially without telling them in advance.