The regulations state that


'The responsible person shall ensure that free tap water is provided on request to customers where it is reasonably available.'

The definition of a customer is 'A person who buys goods or services from a shop or business.'

So, on the face of it, unless you buy something from a pub you are not a customer and therefor not entitled to free tap water.

A little pedantic, but this law is acknowledged to be a grey area (according to my wife who spent many years looking after licensing for our local brewery).

Besides which, as others have pointed out, the licensee does have lots of overheads to cover.