Chatham Historic Dockyard, Kent

After the Dockyard closed as a military base, the boring parts were cleared and re-developed, while the historic bits were conserved and opened as a large-scale museum. Today, as well as interior displays, the visitor can explore a Victorian warship, a WWII destroyer, and a submarine. Inside various buildings is an audio-visual recreation of the sailing Navy, a collection of naval hardware inside a wooden shed which is an exhibit in itself, etc etc. The long rope-making building is still in use. There are many historic buildings on site (not all open). Items from the Tudor navy to the 20th century are on site. Currently (Sept. 2011) components of the dismantled and somewhat charred “Cutty Sark” can also be seen at Chatham. On some days, you will see a steam hauled dockyard train trundling around.
There is a lot to see, and visitors should be aware that it takes some time just to walk from one end of the site to the other. My favourite items were the beautiful Victorian warship, HMS Gannet, the destroyer HMS Cavalier, and the great shed with the complex timber roof, housing a collection of anything from boats to boilers.
(HMS Gannet survived because the hulk was used as a floating Gulag for hapless sea cadets, who were subjected to a regime that today would be considered too harsh even for juvenile delinquents.) Outside the entry booth for the submarine, HMS Ocelot, is a plywood board with a large hole in it, and a sign suggesting that fatties should try climbing through it before proceeding further :-).
Admission prices aren’t particularly cheap, but this is a first-rate museum and the standard price covers a year’s re-admission, which is just as well as there is too much to see in one day. On entry, you will probably be handed notices and invites for timed events, and you could start by making your way to those, and having a preview of what’s en route, bearing in mind that walking to the far end will take about 20 mins even if you don’t linger. Thanks to demolition, there is plenty of car and coach parking. If you have time on your hands after a 2nd visit, the Royal Engineers’ military museum is in the town.