I fish in and around London all the time. I do well on the Thames - whether it be downtown behind Labatts, Greenway park on Springbank, Gibbons Park, or way up around the Rebecca Rd area. (real shallow up there but have pulled out good smallies and a nice little 'eye on topwaters).
I haven't done much fishing in the further west part of town - I hear Springbank park holds some huge carp (not really my fave target), and I've heard that people do well out just off of oxford street, west of hyde park.
We've had some crazy catches too - Big carp - the occasional pike, and a couple of +12 lbs cats all came out of Greenway. There's lots of fun to be had around here.