To try and establish the legal position as I understand it: there is a difference between offering tap water and offering tap water alongside alcohol.

Imagine: coach load of carollers pull up outside pub, all come inside, one buys a pint and rest demand tap water. Errr....no, not obliged to serve them. (Apart from the fact he is not obliged to serve anyone!).

Different coach, thirty people buy a pint and ask for tapwater alongside the pint. Obliged to serve tapwater.

So if you are a driver generously acting as a chauffeur, who doesn't drink what is on offer, wait outside!