The streets of London can present real opportunities for the street photographer. I like to use a "mirrorless", rangerfinder style of camera, to be totally unobtrusive, and have the portability, a camera that fits in the pocket of a coat is ideal. Olympus, Panasonic, Sony, and Fuji make great cameras for this style of photography.
Wet streets make for interesting photos, look at the way that the rain reflects on the cobbles, don't be afraid of getting the camera wet, light rain as this was shouldn't damage the camera for the 30 seconds that it took to take the shot, having set the camera up previously the Panasonic 20mm f1.7 lens lets in sufficient light for effective images, and the LCD screen is all you need to compose the image. The other tip is to get low down, I squatted down with my back to one of the bollards.