Comment by BoxOfRain

The water infrastructure isn't great in the UK (there's a lot of first-mover curses in UK infrastructure generally), specifically there's quite a lot of shared sewage and storm drains. As a result sometimes the local water monopolies end up dumping sewage in the waterways. It'd cost a lot to build enough infrastructure to prevent this, and even if the companies wanted to they tend to get opposed by NIMBYs at every turn.

Also you definitely wouldn't want to treat the Isis (or the wider Thames) because it's a full-blown ecosystem, I doubt the fish would appreciate us pumping it full of chemicals as well as sewage.