That's according to the water companies though, you've taken those figures directly from water UK, an industry run company:

https://www.water.org.uk/about-us/our-board

We must look at the reality more critically.

We already know that the investment is open to profit extraction, the companies doing the work are owned by the same investors and there's low oversight, they have occasionally been caught using inflated prices. We don't know the extent though as OFWAT has been criticized for only really catching the really obvious ones.

All that investment is our own money, it's now apparent they've not taken on debt to pay for that investment. They claim they have, but it's now turned out that they've taken on that debt simply to pay for dividends to shareholders. Debt is around £60 billion, while share holder dividends over the years are about £80 billion (this figure varies depending on who you believe, but it's always higher than the debt). And that's without accounting for the inflated cost of "investment".

And that debt is now apparently tax-payer guaranteed because it's all to people who are too big to fail.

So they've basically made off with £20 billion in 35 years, plus however untold billions in profits from related companies in "investment", and we've got a water crisis developing, constant sewage discharge into rivers, etc.