Information and discussion on all aspects of British photographic history
If you can, you should try and tie in your visit to London when the London Photograph Fair occurs. There are a few held throughout the year, with the first one scheduled for 20th February. You can check the dates of the others in the BPH 'Events' section. It is a highly recommended Fair to go to, with many dealers present, and I'm sure amongst them will have what you are looking for.
In terms of London shops, one that springs to mind is www.shapero.com. They do have a healthy stock of 19th century images.
Good luck with your hunt, and an enjoyable stay in London!
As the organiser of the London Photograph Fair, I would like to echo the comments below - our fair is a great opportunity to meet most of the UK dealers who might stock material of this type. The dates for this years fairs are on our website (www.photofair.co.uk) but if you cannot make it, please feel free to contact them by email - but do let them know where you got their email addresses!