Britain is not suffering a water shortage, nor is it likely to in any foreseeable future. Not that I'm encouraging people to waste water, but conserving water doesn't address the unfavorable economics of energy.
That was the original thought until I went off on the tangent about saving the hot water in a separate tank from the bathtub after it is "used" for the bath allowing the heat to be harvested and the water to be re-used.
The original post is about leaving the hot water in the bath to use the energy to heat the house or help heat incoming water. The idea of an external tank made out of conductive material would eliminate some of the problems and also allow the water to be re-used for the toilet.
Toilet cisterns arguably do reduce usage but then many people complain about less effective flushing which requires additional water; I don't have data to know if these objections are just a cliche or reflect an actual problem. My understanding is that lawn-watering is the biggest waste of water by consumers, to the point that some local authorities will subsidize people reorganizing their gardens to be more suitable for a dry climate.
Now, low-flow toilets have an amazing amount of research and design put into how to meet the guidelines while still performing as well as, and often better than, old 4+ gallon flushes.
A few (ill-informed) complainers aside, this actually seems like a win all around. Toilets don't cost more than they used to, they use less water, and perform better than ever.
https://www.theguardian.com/environment/2019/mar/18/england-...
We have a lot of leaky pipes. I don't get around as much as I used to but it's not uncommon for me to see a different leak every time I go for a walk around SE London.