I also looked through Typophile for relevant stores before heading to London. Aside from Collinge & Clark, the other place for secondhand type books I'm aware of is the Quinto Bookshop. It's on Charing Cross Road, which I would recommend walking along. There are a ton of used bookshops there; at one I found Johnston's Writing, Illuminating & Lettering for cheap. Check out Cecil Court, too; there were a bunch of interesting antique stores there, and I bought a seventeenth-century map there with terrific lettering for a reasonable price.

If you've not been, you should definitely stop in at St Bride Library. A typophile's dream come true.

Just a few blocks north of Collinge & Clark is the British Library. This is worth a visit anyway, as there are usually interesting exhibitions, but their bookshop is also very good and has a whole section on typography and printing.

Between Charing Cross road between Cambridge Circus and Leicester Square tube are plenty of old book stores. Also on this stretch is Koenig Books which sells some nice new books. There are some more bookshops (and general collectibles, cards, prints, maps etc) between Charing Cross Road and St Martin's Lane.

I seem to recall there are some second hand book stores across the way from the British Museum too, which I imagine you might visit.

Also you should visit http://www.stbridefoundation.org/ but check it is open before you go. I think you can view some things there by appointment. Not sure anything is for sale!

Best, enjoy London.

Also underneath Waterloo Bridge (BFI Southbank) are open air book vendors selling second hand books etc.

In Angel there is a small street called Camden Passage that sells lot's of antiques etc, sometimes there are interesting books there. Best to check it out if you are in the area rather than go purposefully.