The water companies are very heavily regulated, both in terms of what they can charge and what capital schemes they must deliver. They are not run as charities whether publicly, privately or private-not-for-profit, so if they are directed to deliver a capital scheme you pay for it. The govt could direct water companies to construct sufficient storage capacity etc to allow people to sprinkle their lawns through the driest summer imaginable, but this costs a lot of money and the customers would have to pay for it. What you end up with in such a case is what they have in the NE - Kielder Water, Britain's largest reservoir, whose water is never touched most years, as it is expensive even to pump it to Newcastle, and it is only needed in the driest years. It was built at a time when the govt thought that industrial water use would grow, but in practice the heavy industry all closed. But at least there was the land and the rainfall for Kielder Water so by the standards of these things it wasn't very expensive, building the equivalent in the SE of England would be hugely expensive. And pumping Kielder's water down to the SE would be more expensive than desalination.

Even leakage control is expensive, which is why the govt regulates how much leakage control the water companies do and how much money they are paid for doing it. Leakage control is more expensive to deliver in heavily urbanised areas, which is why the level of leakage control done in London is less than in other areas. The government determines this, because it is the economic level of leakage control.

Most people would, perhaps, prefer to pay less and have the odd hosepipe ban in a dry year. As I said previously, the major water companies in the SW, Southern and Thames, have applied to build major new reservoirs, but have been refused permission to do so.

A government agency called the Office of Water Services was set up in 1989 and has been considering these things very carefully over the years since then. It simply is not politically feasible to build so much water resource that there is never a water shortage, because it would increase the bills. Much better to encourage people to be more modest in their usage of water.

I expect you will find that the level of leakage from the UK's pipes is low. But it will be hard to verify this, because in the kind of disorganised country where the level of leakage is very high, you just won't get the data.