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The inscription above the entrance still says "Metropolitan Water Board". The Board was founded in 1903 to bring the (then) nine private water companies supplying water to London under a single public body Link .

Under the Water Act of 1973, ten large water authorities were established, based on river basins and catchment areas. In 1974, the assets of the Metropolitan Water Board passed to the Thames Water Authority which covered the area from the source of the Thames in Wiltshire to the Thames Estuary.

In 1989, amidst a flurry of sell-offs of public bodies, Thames Water was privatised and listed on the London Stock Exchange as Thames Water Plc. In 2001, German utility giant RWE bought Thames Water Plc as part of a larger investment into water. The price was £4.3 billion and the company was delisted. Following a refocus of its interests towards electricity, RWE sold off Thames Water in December 2006 to a consortium of institutional investors managed by [Australian] Macquarie Capital Funds (Europe) Limited Link . Over 30% of the stock was sold in 2012. And in March 2017, the Macquarie Group sold its last remaining stake in Thames Water’s holding company. Link

The current ownership and company structure of Thames Water Utilities Limited (TW) is quite simple and straightforward. The principal shareholders were (at 12 January 2018): OMERS [Canada] 27.4%; the [UK] Universities Superannuation Scheme 10.9%; Infinity [Abu Dhabi, UAE] 9.9%; Wren House [Kuwait] 8.8%; bcIMC [Canada] 8.7%; Hermes (BT Pension Scheme) [UK] 8.7%; Cicero Investment Corporation [Chinese Sovereign Wealth Fund] 8.7%; QIC [Australia] 5.4%; Aquila [a wholly owned subsidiary of Fiera Infrastructure Inc. – Canada] 5.0%; Stichting Pensioenfonds ABP [Netherlands] 4.3%; and PGGM (Stichting Pensioenfonds Zorg en Welzijn) [the second largest pension fund in the Netherlands] 2.2%.

The ultimate holding company is Kemble Water Holdings Limited (previously known as Bountydale Limited; and prior to that Britannia Ports Holdings Limited) The next holding company in the chain is Kemble Water Eurobond plc, which is a holding company for Kemble Water Finance Limited, which is a holding company for Thames Water Limited, which is a holding company for Thames Water Utilities Holdings Limited, which is a holding company for the regulated water company Thames Water Utilities Limited. Its financing companies are Thames Water Utilities Finance Limited; and the holding company Thames Water Utilities Cayman Finance Holdings Limited for Thames Water Utilities Cayman Finance Limited. Link (Archive Link )

So the water supply and waste water treatment in London is now over 80% owned by Canadians, the Chinese, the UAE, Kuwait, Australians and the Dutch, and financed (in part) via the Cayman Islands. Link and Link .