Anglian Water encourage Rainwater Harvesting

Why would they do that? 

Well, it turns out that harvesting water from your guttering reduces the amount they need to pump and process. Then if you have water to wash the car and water your plants, it saves them processing potable water and transporting it. Finally, in times of drought, you can remain self-sufficient for longer.

I wrote to Anglian water when I installed three tanks (total capacity of 2,400 litres) asking for a reduction in sewerage standing charges. They welcomed my request, adjusted my bills to 2016, and refunded £253.

I am linking the rainwater via a submersible pump to our washing machine and ground floor loo. The logic is that rainwater is clean enough to flush a toilet and wash clothes. Every litre we save on water reduces the sewerage charge by 0.9 litres, so I will save even more money.

Saving utility charges is nice, but the main reason is to reduce the carbon footprint of a significant energy user and to make the best use of natural resources. The fact that rainwater is free and easy to store is a bonus.
https://www.anglianwater.co.uk/account-and-bill/tariffs-and-charges/sewerage-charges/sewerage-abatements/