Oh it absolutely would. London Below is a brilliant setting and being able to explore that in a game would be tremendous.
I do suppose though that it could make a passable RPG if the protagonist is not an outsider, but one of many characters that inhabits London Below. That might fit the RPG framework a bit more than playing an outsider that stumbles into it all. For example, you could play someone like Hunter rather than Richard.
Hmmm, but then you'd lose all the benefits of having a believable protagonist who genuinely doesn't know anything about the world he's in and is free to discover it.