There's just been a piece on R4 about the River Wandle which in the early 80s was an open sewer and since privatisation massive investment has now made it clean enough for fish to return.
I remember in the early 80s how I used to stop cycling when I reached the bridge over the River Wandle at the junction of Butter Hill and Mill Lane, just to see how many trout I could count. I remember also seeing loads of sticklebacks in the shallows of the Wandle, which is presumably what the Kingfishers fed on. Also plenty of ducks, moorhens, and swans.
The River Wandle in Wallington was very far from being an open sewer in the early 80s. True, I don't really have any experience of what it was like beyond Beddington Sewage Farm, but I strongly doubt it could be described as an open sewer - its course runs through very close to housing. It might well have been a sewer 200 years ago before Victorian investment.
And I'm not sure how kind Thames Water has been to the River Wandle, 5 years ago they killed 7,000 fish in the Wandle, which reversed 20 years of conservation work according the the Environment Agency, when a cock-up during cleaning at Beddington Sewage Farm resulted in massive chemical pollution. The initial fine for that inexcusable incompetence was "thought to be the greatest ever penalty for a single offence of polluting controlled waters".
UK water supplies do have a global reputation for being of high quality, but this is a historical issue which probably has more to do with the legacy of Victorian capital works projects, which saw massive government spending on infrastructure, than privatisation.