Here's a plan that costs nothing at all - cut the wages of public sector employees in London and encourage them to move to wetter places. Then you don't need as much water in London.
Unfortunately the government wants to do the exact opposite, and crowd the South East further. With any luck, that corner of the country will reach critical mass, snap off and sink.
In general, the whole "why should I change my lifestyle - SOMEONE should DO SOMETHING about getting me more water" attitude is pretty hateful. If you live beyond your means in terms of any resource, be it money, water or whatever then either earn more, use less or go and live somewhere you can afford.