Perhaps, in London, the railway station you are nearest to is one indicator. For example, although I live in SE26, I joined this forum (as well as the SE26 one) because we use Forest Hill rather than Sydenham station. (We're also in Forest Hill ward, but wards are, alas, like parliamentary constituencies, purely political divisions and often seem to bear little relationship with real communities. One of the downsides of the principle that the vote of everyone in the country must have the same weight wherever they live.)