I am an ATC "bod" at Swanwick. I used to organise Familiarisation flights for ATC staff prior to Sept 11th and the feedback I received from both the flight deck crew and the ATC staff that access to the flight deck to see the operation first hand was very important and it enhanced flight safety. A few years ago there was a serious incident, and one of the factors in that incident was that the controller did not appreciate a particular aspect of the aircrafts operation with a recommendation that ATC staff have more fam flights to preclude this.
We are all security cleared and have all signed the Offical Secrets Act, but under these new rules we are banned from access to the flight deck