I have to throw in my 2 pence, as having visited the museum and being blown away by the extraordinary kindness by the team at KBBM, my experience there has been the complete opposite.
They’ve had some very serious thefts there in the past, which as has been fairly pointed out, is a problem affecting all museums. If I understand correctly, they’ve also had a number of items for display donated by families of airmen, who stipulated that no photographs be taken of said artifacts. I can fully respect and understand that.
I would implore people to give it a visit; not only does it have a superb collection, but it is the definitive source of Battle of Britain items. 
As said, was very much taken back by the way I was treated there, they were extremely friendly and answered all my research based questions comprehensively.