I don't know. I know a few people who live in green belt areas and commute every day to work in central London. Clearly, in some sense, London doesn't end there

I don't know. I know a few people who live in green belt areas and commute every day to work in central London. Clearly, in some sense, London doesn't end there; and it drives up costs for them and many others. At the same time, I agree that it would be terrible to lose all the countryside vaguely near London. Perhaps your top-level comment was right: London has just reached capacity. But I'm not convinced of that, either; there are plenty of bigger megacities.