I believe walking around is the best way to get the true essence of a city. London is no different. Of course the tube is very convenient and it will get you to most places in town. But, once in the neighborhood of choice don't pass by the opportunity to stroll and photograph the city doors. Chelsea and Notting Hill are great areas for this, but the colorful doors are truly everywhere.
Don't ignore this one, of course. Famous by its occupant and address.