The OP said he doesn't have a meter. so in his case IT'S NOT ALL ABOUT CONSUMPTION. It's based on the historical RV so the size of the house is relevant. The accepted rule of thumb is if there are fewer occupants than bedrooms then you should save money having a water meter (Money Saving Expert).I also do not understand why quoting the number of bedrooms is in any way relevant IF you are on a water meter, IT IS ALL ABOUT CONSUMPTION.
They were until 1989. Since then they have perhaps been reasonable value, but nevertheless more than they coulda/shoulda been, due to the fact that a significant proportion of your bill (typically 15-20%) has been given to shareholders.To be fair, water bills have been very cheap for a long time.
Not to mention companies like Thames Water Utilities Cayman Finance Ltd. and why they were formed!