The population of London isn't much more today than it was in 1930. And the population density is more or less the same too. I think it's unlikely that people's bodies are the cause of heating. Greater London for example would receive about 1572 MW of solar irradiance on a sunny day.
The city burns quite a bit of fossil fuel for heating and transportation every day though. That certainly has effects on microclimates (think vents from the tube, which is getting ever hotter).