London is so well integrated that there's little reason to use a taxi. That's why you have to call to book one, because most people use them for unusual trips, such as going to the airport with a lot of luggage, attending an event with formal wear etc. Much cheaper and quicker to use the tube.
A map of where in San Francisco people succeed in catching taxis in practice, if you want it: http://www.flickr.com/photos/walkingsf/4956860071/in/set-721...
Of course, in London you literally can't catch a black cab outside of the center (they're not even allowed to do so past a certain radius) so you have to call a minicab. So SF still wins.