The Brunel Museum and the Transport Museum are running guided tours of the Rotherhithe Tunnel at the end of May. Tickets have been on sale less than half an hour and are already and absolute dick to try and buy. I think it'll be totally worth it though. Jump on it while you still have the chance:
http://www.ltmuseum.co.uk/tickets?directlink=en-GB/shows/thames%20tunnel%20public%20walk/events