We have done several of the Rick Steves' self-guided walks in his book. I find these to be worth doing in every city we visit.
London Walks does an excellent job. Some groups tend to be too large so if 75 people show up, you might decide if that is going to work for you. No reservations, you just show up. We particularly like them for outdoor itineraries. We used them for a Westminster Abbey tour and then benefit was that on a very busy day with a huge line outside, we were able to skip the line. However, it was difficult to tour the church with so many people and on a second visit during a less busy time-of-year with no lines, we toured again and we thought the audio guide did a better job than the tour guide in the crowded situation.
Last trip we did a tour with Brit Movie Tours which was fun. they have several itineraries.