Very nice job. I don't have a use for it since I'm not in London, but I like your implementation and design. A comparison feature would be nice, so if you're uncertain where you want to go one day, you can compare activity.

You might also want to take a look into incorporating Spotrank data (http://www.skyhookwireless.com/spotrank/examples.php).

SimpleGeo is one way to do that (http://simplegeo.com/).