If we want cleaner water we are going to have to pay for it, the regulator is OFWAT, they decide how much is spent on the water industry in the UK.
Successive governments have instructed OFWAT to hold down water bills, that has been the prime cause of poor water quality.
Management of the water companies has been poor, however don’t believe that nationalization will be a quick fix, my guess is if water bills were doubled there would still be a lot of sewage spills. Modern construction separates storm water from sewage, in older areas storm water overloads the treatment works, separating the 2 systems would help a lot.
As for swimming in rivers, the official advice is that inland fresh water is not safe to swim in in the UK, it contains all sorts of bacteria and parasites, that’s not going to change any time soon. By the time the Thames has reached London it’s been flushed 6 times, swim in it if you want I won’t be joining you.