Other than the usual stuff (which I won't go into e.g. Tudor England, the V&A, HOuses of Parlament, etc.):

IF you are a Beatles Fan, you have to go to Abbey Road (and cross the street...if you are Paul Fan, take your shoes off!)

The two times I've been in London, I've done walks with the following outfit http://www.walks.com/ I've done the WEstminster Nobody Knows Tour and the Saturday Explorer. They have seemed to have added a lot more in the last couple of years! I've always wanted to to the Jack the Ripper ones and the ghost ones (these happen at night and stop at a few pubs) but never got the chance! Next time!!

Perhaps travel cannot prevent bigotry, but by demonstrating that all peoples cry, laugh, eat, worry, and die, it can introduce the idea that if we try and understand each other, we may even become friends ~~M. Angelou~~