According to David Cameron, London is a floodplain.

This from Miles King:

[The PM] rejected the idea that it was a bad idea to build on floodplains claiming that London was a floodplain. Only a tiny proportion of London sits in the floodplain of the Thames, perhaps he was confusing Westminster with London.

The story of the Voles, the ditch and the Prime Minister
https://anewnatureblog.wordpress.com/2016/01/13/the-story-of-the-vole-the-ditch-and-the-prime-minister/