From their website:
Sub meters
Having a sub meter fitted to an external supply like a garden tap can also help reduce your bill. Because the water you use in the garden doesn’t go back into the sewers, we can adjust the amount you are paying for sewerage services once you provide us with a reading from the sub meter.*
*We don’t supply or fit sub meters, so if you would like one you will need to arrange for it to be fitted and supply us with the readings.
https://www.anglianwater.co.uk/account-and-bill/tariffs-and-charges/sewerage-abatements/
The sub meter can be a simple unit which just connects to your external tap,
or a more permanent fitting, plumbed in to the external feed.
Prices range from around £20 to around £40. (Search "Water Sub Meter" on Amazon).
18-06-21