It's a long way off yet of course but when this starts to become a more pressing issue, a couple of things will happen. First, enhanced sea defences and other such projects would see London receive an even more unfair share of taxpayers' money as we all tip our pockets into the Thames estuary to yet again protect those poor, precious financiers. Then, when the government eventually comes to terms with the fact that London and the 'home counties' cannot be saved, the higher land to the north will start to finally look attractive, despite the accents, flat caps and post-industrial deprivation. When that happens, there'll be an almighty clamour for the most appealing locations and those who already in those areas there will likely sell up, make a tidy profit and set up home in the next nice spot. Non-Londoners might even start to reclaim money they've lost to the capital over the years.
Of course, this potential future assumes that governments and commercial inhabitants of the south-east continue to blindly hold up London as our shining symbol of greatness rather than do the sensible thing and set up shop somewhere cheaper and more sustainable. But given the lack of farsightedness and pig-headedness displayed by politicians in power and their privileged patrons for most of the last century at least, I don't think it's too difficult to imagine.