According to the Environment Agency reported on the BBC UK website yesterday:

[In England] there were 56 serious pollution incidents last year [2018], rising from 52 in 2017, the agency’s annual report said. Only one of the nine major water companies in England is performing at the expected level, with most likely to miss 2020 targets, the agency added. The report follows the agency’s announcement that Southern Water is facing prosecution after it was hit with a record £126m penalty package over “shocking” failures in its sewage treatment sites.

https://www.bbc.co.uk/news/uk-england-48934421

Meanwhile in Scotland, according to SEPA

In 2017, for the first time in a number of years, Scottish Water was not responsible for any category 1, serious pollution incidents.

https://consultation.sepa.org.uk/sector-plan/water-waste-water-treatment/user_uploads/water_supply_and_waste-water_sector_plan_draft_annex.pdf

A report in Open Democracy attributes the situation in England to privatisation:

The dire state of our rivers is just one of the many, many failures of water and sewerage privatisation. It is just one example of what happens when we hand over not just time-limited contracts for delivering water and sewerage services, but the actual assets themselves – the pipes, the infrastructure – to private companies to milk for profit.

https://www.opendemocracy.net/en/oureconomy/when-we-own-it-public-ownership-water-21st-century/