I have to believe it is possible to write a story about a place you've never been, otherwise I'm in trouble. But it IS a lot easier if you've been there. Sometimes you just know an author has been to a place by the incredible depth of their descriptions.
The best advice (short of flying to London and spending a week here) would be to do lots and lots of research. Books are good, movies are good, maps and Google are great, and Google Earth is my new best friend for checking out what a place really looks like. And if you really want to be sure, run your story by someone who HAS been there when you're finished- just so they can catch any mistakes.
And for the record, lots of people in London (especially East London) call each other "mate." I got called "mate" just this morning.
A Mad Tea-Party