I have to say a pretty rare luxury to find here also and those without a big estate but plenty of money may attempt to build a permanent one but often end up with all pool and no garden (backyard).
Then there are the filtration pumps and the chemicals to provide, and the expensive solar panels to install to produce some power for creating a bearable water temperature ..... then the roll-out cover to stop it freezing in winter and to collect the falling leaves of autumn.....then count the number of days per annum anyone actually swims in it ..........
I think I will just stay with the dreams of David Hockney and his Bigger Splash in CA (he's come home to Yorkshire by the way - quite a contrast)