I have recently seen references to the Science Museum storage facility at Wroughton and, given that I've been to a fair number of aircraft museums around the UK and never even heard of the place, I thought I'd check up on accessability.
The following link is to the events section and gives details of when the collection is open.
http://www.sciencemuseum.org.uk/wroughton/diary.asp
Hope this is useful.
SD