Here's the most ridiculous (private) mansion in London, though. Just off Knightsbridge in Rutland Gardens, close to Hyde Park:
https://goo.gl/maps/iZp5m
It was on the market a couple of years ago for £300 million ($494 million):
http://www.bbc.co.uk/news/business-19571104
Note that either Google Maps has the address in the wrong place or the Daily Mail posted photos of the wrong building (on the other side of Rutland Gate), but they are very similar buildings either way.