I decided on "About A Boy" because not only is it a great movie, it's also a great book by Nick Hornby. It's one of those movies that you always watch when you come across it on t.v. Ironically, it was directed by a pair of American brothers, Paul and Chris Weitz, at the time more famous for the "American Pie" movie.
It's a great London film because it's not overly in your face London. Most of it was filmed in North London...Regent's Park, Clerkenwell and Kentish Town and a bit in Notting Hill and Waterloo.
This is the movie where Hugh Grant stopped playing the floppy haired bumbling Brit and started playing the short haired kind of a jerk Brit. Also the boy, Marcus, was played by a very young Nicholas Hoult...who has grown into a very attractive man.
Another reason it's a great London film is because it perfectly sums up life in London. How your 'family' and friends are not always the people you expect or want but they are usually the people you need.
It also has a brilliant soundtrack by Badly Drawn Boy.