Rather than taking place in only one part of the city, Stevenson’s novel is set in many different areas of London, each meant to reflect the character of its denizens. Soho, where Mr. Hyde lives, for example, is described as being dark and dingy, which, last time we checked, tends to go quite well with people that ooze evil out of their pores. London fog also plays a role in the book, adding sinister overtones where Stevenson needs them.
We start off with the story of a door in a rough neighborhood that leads to a passageway that leads to Dr. Jekyll’s laboratory. Think of it as a passage between two worlds. Until the end of the story, Jekyll (himself and as Hyde) is the only one who traverses this passage. He’s also the only one walking the fine line between normality and evil. Sounds like a symbol to us.
London Fog vs. Light
The London fog serves to shroud or veil the city and make it eerie. Fog = obscurity, and the literal fog emphasizes the metaphorical fog surrounding the true identity of Hyde. You’ve also got firelight, lighted lamps, and light in general as the counterpoint to fog because of their safe, illuminating qualities.