The use of water meters was ...

The use of water meters was not made compulsory in the Uk because of the fears that the poorer people would avoid using it because of cost.

In the 19th century the real concerns were for contamination of conventional water supply e.g. rivers, wells etc. The 1854 cholera outbreak had a good deal on influence on sewage but it also lead to greater investment in potable water supplies including the Royal Family's Sandringham estate which supplied the locals (the King or Prince of Wales was also infected). https://en.wikipedia.org/wiki/1854_Broad_Street_cholera_outbreak

As to the use of dual water, those with metered mains supplied water still continued to use rain water for other uses. There are a number of studies made in the 19th century which are detailed in a paper given to the Institute of Engineers in 1882 By J J Tylor which is to be found on the internet. Over time people have come to use potable water for everything from washing and bathing to watering the garden and filling fish ponds. OK when their is adequate supply but we also see more use of interceptors in rainpipes to gather rainwater for garden use.  Rainwater is probably as clean as it gets. Ground abstraction licences (in the UK) are strictly controlled and new ones hard to get.