It's easy to work out the breakeven point if you can get the exact metered rates from your supplier.
Anglian Water, for example, charges as follows:
Supply £24.00 per year plus £1.0243 per cubic meter
Sewer £32.00 per year plus £1.1548 per cubic meter.
In plain English, a cubic meter is equivalent to approx. 264 US gallons or 220 Imperial gallons.
Compared with the unmetered charges of £148.10 for supply and £145.46 for sewer, that gives a breakeven point of about 65 Imp. or 78 US gallons per day. If average usage is below that, then metered works out cheaper.
If I remember correctly,when water meters were first introduced on the Isle of Wight,people soon discovered that they were being charged for the water as it came in and as it left,many started emptying their baths through the window,throwing washing up water on the garden etc.
They were wasting their time. The bill does contain both supply and sewer portions, but they are
both based on the amount metered on the supply side. Measuring the flow on the sewer line is just not practical. Houses with their own septic tanks escape the sewer portion of the charge, of course.
It's also worth mentioning that many utilities make a separate charge for surface-water drainage. Anglian Water, for example, charges an additional £30 per year on both metered and unmetered bills.
This amount is added automatically as a default, but if you can demonstrate that you have no surface-water draining into the sewer you can have this charge removed.