I don't have a family of your size, but I know water meter standing charges vary massively.
I know tenants who have installed meters here (without telling the landlord) to go get their bill lowered and found their bill has gone up not down because the standing charge is quite high so it's not only usage you need to be wary of.
We moved into one with a meter, we were really worried but it's been far cheaper, we were paying £60 in old house and now it's more like £25 a month. We don't have as many dc but have lots of baths and showers, do lots of washing etc... In South Wales.
The cost depends where you live. I live in the South East in a Southern Water area and are a family of six we pay £83 a month on a meter (installed by the previous owner who lied on the solicitors form) so about £996 a year.
I can't get it down as we had an advisor come round who was stumped as we have low flush toilets, short showers, never water the garden, wash the car, use eco cycles on appliances etc. They stuck a few water bubblers on the taps that no one likes so something was done. Our usage is apparently below average for family size.
My neighbour pays £695 a year on a rate and is profligate (sprinklers on all day in the heatwave etc) makes me a bit mad. I think only single people or couples have any chance of paying less on a meter than on an RV.
Ours went up from about £25 to £40 a month when we were compulsorily metered. I try and console myself that we were getting it very cheap for many years rather than expensive now. My neighbours aren't metered (technical reasons) and have had their sprinkler on for hours on end through the hot weather .