Place names in London are mostly very old and have changed over time. Their pronunciation often does not correspond to what we would expect from the spelling. For example, some syllables are completely lost, or for some unknown reason "s" is pronounced like "g". Vocabulary is often "swallowed", for example the common ending "ham" is pronounced more like " uM".
Here I would bring the pronunciations of some of the most common names as sound samples so that you can say these "like a real Londoner" when you go there. Of course, keep in mind that many different dialects are spoken in London, and what is presented here is not the only possible way of pronouncing these names.