You've already spotted the Photographer's Gallery which is always worth a look.
The Victoria & Albert Museum has a small gallery dedicated to photography. The Tate Modern might have something on. I think their next show is Francis Bacon who used photography as a reference for some of his painting.
The Magnum Print Room is worth a visit just to feel the energy of the place.
The Stephanie Hoppen Gallery is showing work by Martin Osner.
Grab a copy of one of the free papers they give out on the underground. They have an exhibition listing in the back.