Chances are, your waiter/waitress might not be clued up on Paragraph 2 of the Licensing Act 2003 (Mandatory Licensing Conditions) (Amendment) Order 2014, but the manager should be. So, if you're refused free tap water, ask to speak to them.
If the manager refuses, you can contact the licensing department at your local authority (check the 'contact us' section of its website), which should then investigate. The maximum penalties for breaching a licence condition in England and Wales are six months in prison and/or a fine of up to £20,000, but it's more likely to prompt a review by the licensing authority, which could result in revocation of the licence to sell alcohol.
The penalties for non-compliance in Scotland aren't quite as strict. The establishment's licence can be either revoked, suspended, or varied following a breach.