In the UK, your water supplier cannot be changed or ‘switched’ like your energy supplier can.
The easiest way to find your water supplier is via the acasa app; go to “Services” and then “Water” which should show a link to your current supplier.
You can also find out who the supplier is via this Water UK search tool.
If that tool is not showing your water company, it’s possible your address is too new to have been added to the water companies’ databases yet. In this case you could search for a nearby property’s postcode and contact their water company to check whether they have you in their system.