Water is provided by regional monopoly suppliers, as you have realised.
"The government wants to make switching happen. It's asked the water regulator Ofwat to provide an assessment of the costs and benefits of making switching possible by the summer. It says it will then work with water companies to get household retail competition started by the end of the Parliament - so by 2020.
There might be a real appetite for it, given the Public Accounts Committee reported in January that households have been paying too much for their water supply because Ofwat has been over-estimating their costs and, consequently, price limits.
Switching suppliers?
However some of those in the know think water switching is not going to work. Switching would only bring down the costs of the retail element of your water bill, which covers things like customer support and billing itself. The supply and treatment of your water would stay with the incumbent provider in your area, and that is the main cost.
The Consumer Council for Water reckons that customers will typically be able to save less than £10 by switching. And it might be instructive to look at large non-domestic users of water, like businesses and charities - they can switch, but only four of them have."
https://www.bbc.co.uk/news/business-35787379