I've pinched this off Subbrit but on a general note some of the places open during the Heritage Open Days of 6-9 September are well worth the effort in seeing as a lot of them are normally out of bounds to the public ; http://www.heritageopendays.org.uk/
As part of the Heritage Open Days, the restored Fighter Command No.11 Group Operations bunker underneath the former RAF Uxbridge will be open to the general public on three days next month (Sept).
Normally only open to guided tours, this is chance to go inside if you haven't been before without needing to arrange a special tour group.
A number of military sites across the UK will be open - I suspect the Greenham Common decontamination centre will interest some.
Select Military from the "Type of venue/event" drop-down menu.