Hmmm... let me see...
Cambridge- Heffer's: for many years Cambridge's only large bookshop, and still catering for mainly for the academic and reference market. A fairly ordinary shop front, leading back into a much larger space with four levels - a sort of bibliophile's cave
- Waterstone's: larger than Heffer's, on four floors. The SF and fantasy is in a quiet area at the back of the ground floor, and they even provide comfy chairs
London- Stanford's: this one is the UK's best-known specialist map seller, and I could browse there for hours.
Oxford- Blackwell's: the Oxford equivalent of Heffer's, with the same sort of layout - small on the outside, huge on the inside. I've not explored all the side passages and basement levels, but it could easily be the local portal to Pratchett's L-space.