I hate having to buy mineral water at restaurants. I'm perfectly satisfied with a glass of tap water.
MoneySavingExpert.com has produced an excellent guide to getting free tap water in restaurants. It seems that
...only licensed establishments (those that serve alcohol) in England, Wales and Scotland must provide free drinking water.
According to Section 136 of the Licensing Act 2003, which came into force in April 2010, all restaurants in England and Wales that serve alcohol are legally required to give customers free tap water (the legislation for Scotland is the same, but it's a different act). Those that don't are under no obligation to do so.
That doesn't mean that they can't charge for the use of a glass!