Lived all over London for 20 years, including pre-gentrification Hackney and very gentrified parts of Lambeth and there’s never been anything like that.
Not even in Croydon, and they’re feral.
If I was to indulge this for even a second I’d point out this massive project sewage project involving 6 tunnel boring machines: https://www.tideway.london/the-tunnel/
Meanwhile, other nearby countries that are EU members still have urban areas with no sewage treatment plants at all and just dump all their sewage into waterways in normal operation, something that's against EU rules and I'm pretty sure was stopped a long time ago in the UK. That doesn't get as much media coverage because it doesn't fit a useful narrative, though, to the point even people from thsoe countries are convinced their country is doing a better job.
Also, this story largely originated in the Guardian, and it turns out prior to Brexit they were pushing the narrative that the big megaproject to 95% fix this in London was completely pointless and only served to channel cash to the Tories' pals, the problem wasn't really that bad and the overflows were mostly rain water anyway: https://www.theguardian.com/environment/2014/nov/27/london-s... This is, uh, rather different from the current narrative they're pushing.
That makes it seem like the UK press is largely left leaning, when most of it is very much right leaning and usually pro-government in sentiment.
The Mail has far more readers than the Guardian, for instance. The Sun more than the Mirror. The BBC, while historically left of centre, is nowadays pro-government after threats of funding cuts brought them to heel.
There are a lot of reasons beyond Brexit to think the government is doing a poor job, though Brexit itself is obviously a huge point of contention.