If you are looking for a guided walk in London, I can highly recommend Footprints of London, the company I work with.
Footprints of London is a London guided walks company with a difference: It is owned by the guides themselves. All the guides are qualified and accredited; and all the walks are researched and written by the guides leading them.
Walks normally cost £10 each (£7.50 concessions) and have to be booked in advance – see:
Have a look at this youtube video
The range of walks is huge, so do think about choosing Footprints of London if you are looking for a walk in London