There is a charge for disposal of water from houses. The charge is based on
the amount of water supplied to you (or estimated if not on a meter). If
you can show or give evidence that the rain falling on your roof does not
enter the sewers, the water company only charges for supplied disposal. If
your subsoil is suitable, have a soakaway dug under your lawn - any builder
will advise.

You could always reduce your water bill even more by having rainwater stored
in a roof tank to be used for flushing toilets but it will mean capital
outlay so might not be much of a saving but if costs for water rise . . . .
.. .

Geoff