Comment from a non-Londoner: it strike me that the difference between London and other cities, in terms of getting out of it (or even into it), is not just its size but its "sprawl gradient". Almost every city has outskirts and suburbs which once had separate identities but there always comes a point where you can say This is (currently) the last building in this city; beyond here is fields, woods, heaths, whatever, until the next lot of buildings just over there, which is a different town/village/whatever. But London doesn't seem to have any such point, at least not in any direction I've ever entered it from. It just has miles and miles of suburbs which merge into the next identifiable separate town without a break. It's not so much that there is no greenery but that the greenery is not a place.