That's a nice idea re using an existing place as a starting point. I think some users won't have a clear idea as to exactly what they want so making it easier to "browse" could be helpful.

For the most part extending to the whole country shouldn't be too difficult (famous last words!) -- most of the data sources I'm currently using aren't specific to London. But I think I'll probably first go deep rather than wide, allowing the user to look at areas at a more granular level should they wish to.

Politics is on the todo list as well!