Investment by water companies is quite high. There's a huge industry built up around building and maintaining process plants as well as infrastructure construction. I've been working in process plant construction for the last decade and a bit. One of the big things at the moment is waste to power, creating electricity from sewage.
The industry regulators set the budgets for new assets for each water company and if they don't spend the full budget within the 5 years then its cut for the next 5 year cycle. What they always do is give the entire budget of billions to investment bankers who then buy and sell shares with it for a year. They then tell us that we have 4 years to do 5 years work, or this time we only have 3 years because of covid for some reason.
One of the major reasons for discharge of less treated sewage at the moment is that brexit has interrupted the supplies of ferric sulphate which is critical for sludge removal processes.