Fitzrovia

 was an area in London.

It had narrow streets and was in the area of the BT Tower and a museum where Polly saw lockets with miniatures of Charles I and Oliver Cromwell, as well as one miniature of Christopher Whyte. (PROSE: The Roundheads)

Behind the scenes

 * Polly, Charles I, Cromwell and Whyte are not named in the prologue scene of Polly visiting the museum. Neither is the museum, while the BT Tower is simply called "the Tower". However, all but the museum are recognisable based on the context of the book and of the TV story The War Machines.
 * Given that the BT Tower is an actual building in the district of Fitzrovia, a lot of filming for the The War Machines was done there.