The Underground tours!!! So cool and spooky. Not sure about this particular company, but in general totally worth doing. Underground opium dens, prostitution, speakeasies and shanghaiing (see : http://en.wikipedia.org/wiki/Shanghaiing).
Dover Castle, Kent - one of the best castles I have visited and so much history.....used through WWII for many different purposes......there are 4 miles of underground tunnels that were used as a hospital during WWII.....built on the White Cliffs of Dover