On a recent trip to London, Carol and I stopped by Stanfords to browse their wares and found ourselves captivated by the huge selection of maps, travel books, CD/DVD packages for mapping, software, GPS devices, globes, and other memorabilia.
Although we didn't see everything in the shop, I can completely understand why a Londoner who was planning a trip would stop at Stanfords first to prepare. While there you can browse their maps and books, or if you need specialized maps for your hike or group outing, you can talk to one of their specialists and produce a large-format map using one of their giant plotters (think three feet or more wide).
No matter what you're looking for, it's a fun place to browse and check things out. One caveat: they don't take American Express, much to the chagrin of corporate buyers from the US.