Choosing The London Underground as your fancy dress party is a great idea. 
For a huge amount of London Underground Costume Ideas, take a look at this information page, it has everything you, and your friends, could need for your costume inspiration. Why not share it on Facebook, or print it off and hand out in your invitations to encourage more people to dress up?
Today in 1907 the Charing Cross, Euston and Hampstead Railway (CCE&HR) opened as an underground railway. Later in the 1920s, this line was connected to the City and South London Railway (C&SLR) but because the initials were getting a little out of hand, it later became known as a major part of what we now know as the Northern Line. Here in Essex, with Ongar as an outpost of the Tube network, we occasionally get the ‘Come Dressed as the Name of a Tube Station Theme’ crop up. For some it is not too much of a problem – Angel and Blackfriars are no-brainers, but after that you might have to start thinking creatively. This is one situation where a theme’s outfits – ‘Tube Station Costumes’ – don’t come off-the-peg on the Internet (although there are a few chat rooms with suggestions). You therefore might need to come to somewhere like ourselves for help (or at least consult our specialist information sheet on this very topic, to be found elsewhere on this site).