Would highly recommend any of the London Walks that take your fancy. They start I think either at 11am or 2pm from usually outside a tube entrance. You don't need to book so just turn up and pay the guide. They last about 2 hours. We have done the London in the Blitz [around St Pauls], Inns of the Court, and the Palaces of Westminster. All great and some of the guides are sometimes actors in between engagements who out on a bit of a show. You can look them all up I think at Londonwalks.com