If you are in the Thames Water region and are currently on water rates then Thames Water will fit a water meter for free if you would like.
The good thing about this is that you can actually decide within 12 months of having it installed that you don't want to be charged from the readings on the water meter and instead just pay the rates value you were paying previously.
It's sort of a can't lose option if you think you may be able to save money by having a meter fitted.
I know that we pay less on a water meter than we would for rates - that's with 2 adults using a power shower daily and my daughter having a bath. But we don't water the garden! (Which I'd imagine would use loads of water).