Suppose you are one of London's 72 resident billionaires, and you're making 5% or so on your income.

You can pay for this digger in an hour, by spending that hour sitting on your ass, and it's not that you will be re-spending that money in your lifetime anyway. Fill it in or have someone salvage it, do whichever is quicker: you've got other things to do with your life and one of those things is to enjoy your new pool.

(Whether true or not, this acts as a good illustration of the scale of income inequality)