I also visit London quite often and I have to admit it's one of the few cities I feel safe walking around alone at night in most neighbourhoods.
Depending when your free time is, do remember that access to most musuems is free so if you can make the time to head to the Natural History Musuem, there is some seriously nice photography to be taken in there... also the building itself is real nice at night. The same is true of the Victoria and Albert musuem and many others.
Hyde Park is also well worth a look, the Albert Memorial (and the Royal Albert Hall across the street) make for interesting night pics. The Kensington Gardens in Hyde Park may also make for interesting pics.
A note, remember that tripods are not permitted inside most musuems
(I was warned...)
Pat