I wrote about setting scenarios outside London (more generally, outside the major city of the region), and thought it was broad enough to cross-link here.
Looking back, it's a bit brief. For example, I dismissed the idea of exploring London, and I think there's something to that: if you're actually engaged in a scenario, in-character tourism is likely to limit itself to a few major landmarks. I know there's a glamour about the city for some people (Americans in particular, I suspect) but most of it is just, y'know, streets and things.
But an obvious flipside is familiarity-tourism. If players are familiar with London, then they may well get a lot of enjoyment from immersion, and so being able to wander through familiar districts or simply get the appropriate Tube lines can be a source of pleasure. And having those things handwaved might dispel the sense of immersion.
You have to weigh this sort of thing up. Broadly speaking, I think a scenario aimed at creating a Being In London experience does need to concern itself with verisimilitude. Cthulhu by Gaslight and suchlike games tend to fit into this category. However, a scenario that's set in London as a generic default doesn't, really. If London is simply a convenient place to nominally be, and all the action happens in named locations, then it's fair enough for the GM to skimp on it.
I think it's still true, though, that the smaller the settlement, the more significant each person and place becomes. In a hamlet, each of the six residents is significant, and the presence of a bridal shop is notable. In a city, there's going to be a bridal shop somewhere, so why not here? And we have to take it as read that there are far more NPCs in the place than players can possibly interact with, almost all of whom are irrelevant in the extreme.
And I do still plan to get back to that scenario, and rewrite it completely as a setting book with a couple of scenarios built in...