Well, I also live in the Mission next to Valencia, where it is ludicrously easy to catch a cab, though I appreciate that it's impossible to catch them in, e.g., Sunset, so I didn't mention it as an advantage.
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.
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.
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.