As in Clockwork Angel and Clockwork Prince, the London and Wales of Clockwork Princess is, as much as I could make it, an admixture of the real and the unreal, the famous and the forgotten. The Lightwood family house is based upon Chiswick House, which you can still visit. As for No. 16 Cheyne Walk, where Woolsey Scott lives, it was at the time actually rented together by Algernon Charles Swinburne, Dante Gabriel Rossetti, and George Meredith. They were members of the aesthetic movement, like Woolsey. Although none of them were ever (proved to be) werewolves. The Argent Rooms are based on the scandalous Argyle Rooms.
As for Will’s mad ride across the countryside from London to Wales, I am indebted to Clary Booker, who helped me map the route, found inns that Will would have stayed at on the way, and speculated on the weather. As much as possible I tried to stick to roads and inns that did exist. (The Shrewsbury-Welshpool road is now the A458.) I have been to Cadair Idris myself and climbed it, visited Dolgellau and Taly-Llyn, and seen Llyn Cau, though never jumped in to see where it would take me.
Blackfriars Bridge exists of course, then and now, and the description of it in the epilogue is as close to my experience of the bridge as I could make it. The Infernal Devices began with a daydream of Jem and Tessa on Blackfriars Bridge, and I think it is fitting that it ends there too.