As an ex area-manager for a large chain of bars, we used to advise against it. You'll get eithe rpeople taking up space with a glass of tap water or people drinking too much, having a glass of water and then vomiting everywhere.
Flat out wrong.
From the Home Office guidance on the mandatory licensing conditions (which helpfully sorts out 'reasonably available' for you too - no QC required
):
Section 2: Potable water
The 2014 Order provides that the responsible person must ensure that free potable water is provided on request for customers where it is reasonably available.
Many premises already offer free tap water. This condition means that responsible persons at all premises must ensure customers are provided with potable (drinking) water for free if they ask for it. It does not need to come from a tap. This helps people to space out their drinks and not become intoxicated quickly, which reduces the risk of crime and disorder occurring.
What is meant by “reasonably available” is a question of fact; for example, it may not be reasonable to expect free water to be available in premises for which the water supply had temporarily been lost because of a broken mains supply and where no alternative (for example, bottled water) is available.
In this section, the term ‘customer’ has its plain English definition.
So basically - if you serve alcohol, you have to have drinking water available for free in normal conditions. Since 2014, any licensed premises refusing is in breach of the law, no matter what their area manager advises...