The original London Walks company has walks for just about any subject on any day. Reasonably priced walking tours that explore neighborhoods such as Westminster or Hampstead, or people, both real and fictional such as Dickens, Shakespeare, The Beatles, Harry Potter or Sherlock Holmes. There are also themed walks including pub walks, foodie walks, and mudlarking along the Thames.
There's no booking in advance. You show up at the designated Tube station exit, look for a guide holding up the brochure, pay and walk. Most walks within London run about 2 hours. The beauty of it is you can decide on a whim--if it's raining, check the schedule for an indoor tour, like the British Museum or Westminster Abbey. If it's a gorgeous evening (or maybe if it's foggy and cold) try the Jack the Ripper tour!
Be sure to visit the website for all the details.
I'm having a hard time choosing!