The Internet is a tremendous boon to writers who are using settings they've never actually visited. Here are a few tips for finding out as much as possible about a setting, going beyond the usual guidebook stuff:
* look for web sites that will introduce you to interesting places other than what a tourist might typically see. One example for London locations is www.uniquevenuesoflondon.co.uk, which features a dozen or so unusual places for meetings, including 195 Picadilly, the home of the British Academy of Film and Television Arts, pictured;
* look for photos on Flickr and other sites that feature shots of locations from all kinds of angles and positions;
* look for blogs on which people describe their visits and impressions of the locations you have in mind--these are a great source of the kinds of details that make a place come alive on the page, especially information you can't get from pictures alone (smells, sounds, temperature, etc.).