Myretta has put together a fabulous resource for understanding where all the real and imagined places are in this novel.

The London page is linked in a thread below, but I wanted to make sure this does not escape anyone's notice. The entire S&S Gazetteer includes the rest of England and even the East Indies (so you can see where Colonel Brandon was stationed). This has many hours of interesting reading and really helps you to picture where things are in relation to each other--such as why it took them 3 days to get to London with Mrs. Jennings, why they can't just go back to Barton 'tommorrow', as Marianne wishes, where Bond Street was in London, etc.