Not sure how the English handle this kind of taxation but in the US most municipalities would spread the cost of a project like this over all users or add as some assessment to property taxes.

In other words, not just “rich people” would pay for it.

There are still quite a few otherwise modern cities in the US that also have this legacy problem (shared sewer + storm drain). My guess is that the cost is considered to be far too high to rip up and rebuild these systems.

If the costs are high, then raise taxes - progressively - to rebuild these systems.

no one wants taxes to raise. poor, middle, rich, you name it. because it just means less money at the end of the month.

If my taxes go up 1% while those making $1M+/year go up by 10%, in order to get improved sewage treatment systems and more, then I'm all for it.

While rich people tend to use their free time for something beside going to the local English beach for a swim.

Universal single-payer health care is cheaper and with better overall health benefits than the current US system has. I would gladly pay higher taxes than pay for my own health insurance as it means more money at the end of the month.

How much money should you get at the end of the month for letting the water be more polluted with untreated sewage?

Promoting single-family homes, prohibiting detached housing, zoning laws that require parking and minimum lawn sizes, etc. all drive up the cost of housing for people who don't have as much money.

It's also cheaper to maintain infrastructure (streets, water, sewer, etc.) in older more densely packed (and also less expensive / poorer) neighborhoods than in more expansive suburbia. As https://www.strongtowns.org/journal/2018/5/7/the-more-we-bui... puts it "The poorest neighborhoods are, in essence, subsidizing the rich ones."