I have been on five walking tours in London, and here is a brief recap:
London Walks - Old Westminster by Gaslight; evening tour around Parliament, Westminster Abbey, St. James's Park and selected back streets. Loaded with interesting tidbits about history, architecture, politics, fashion and yes, the gaslights in the area, some of which are original and still working. Rating: A+
Insider London - Underground and Tube Tour; learned about the history of the London Underground from its inception to station architecture to the distinctive logo and system map by visiting multiple stations. Even got a glimpse of an abandoned station! Need to reserve in advance via their website. Cost is 20 pounds.
London Walks - The Beatles "In My Life" Tour; loved seeing where the Beatles played their impromptu concert on the rooftop, where John met Yoko, where Sir Paul's office is located and more. The tour culminates with a visit to Abbey Road Studios and a photo op reenacting the famous Abbey Road album cover.
London Walks - Jack the Ripper; considered a classic walking tour, it weaves the gory tale of Jack the Ripper and his victims with historical information about life in 19th century London.
Ghosts, Gaslights and Guinness; long, boring tales of haunted sites and ghosts. Found myself waiting for the punchline on most of them. Didn't help that the guide's affected story telling was difficult to understand and I wasn't dressed properly for the cold, damp evening. Friends went on the Ghosts of the Old City walk and raved about it.
When I was trying to sort through all of the tours, I found that some might have different names and schedules, but basically cover the same area/information. For example, there are multiple Jack the Ripper tours, multiple tours of the Westminster area, multiple Old London tours, multiple ghost tours, multiple Beatles tours, etc. Also, I was able to eliminate site-related tours (British Museum, Westminster Abbey, St. Paul's Cathedral, etc.) as I used other methods such as site-supplied audio guides. In addition to London Walks, I recommend you check out the offerings at Insider London.
I hope this helps. Remember to dress appropriately for the weather, be prepared to walk/stand for about 2 hours (standing was the hardest part for me), and have some spare pounds handy to tip the guide.
Have a great trip!