I immediately wondered about planning permission for the pool house. But looks like it's been there for years so beyond enforcement procedures now.
Odd sloped plot with house built into the ground, then teh pool house roof is low (to avoid annoying the neighbours) so you'd have to watch your head coming out of the pool.
I wouldn't be surprised if it was bought for development potential into a B&B or HMO. The pool area would be repurposed into living accommodation.
A shame. The pool could be made and the roof made retractable. It's be a great party room!