Mick - why do you object to the "lock-in" as such?
What does it mean anyway, just that you can't leave half way through the briefing (motive being to try and be first to break the news perhaps)? Sounds like simple good manners not to do this anyway and stay and glean the whole story from the MoD perspective.
Plus it maybe just a sensible way of controlling a large group of visitors in Main Building?