Serpentine Boats, Hyde Park. Boat Hire in London's most famous Park for less than £10.Regents Park Boating Lake. Similar to Hyde Park, and just like in the end bit of that Happy Go Lucky Film.
Alexandra Palace Boating Lake, N22. In the shadow of the palace itself, and bound to be reasonably priced. Plus it's next to the (pitiful) deer park and you can do a nice countryside walk to Hampstead Heath.