Slight thread drift, but since it's been mentioned - people bringing in their own bottled water or soft drinks - it ain't on, unless perhaps you are buying a drink from the bar too, and just taking a wee gulp of water to improve your singing voice between songs.
There was one famous occasion at a certain festival where the barman ordered "everyone out" as they'd all been treating the licensed venue as if it was BYOB, and bringing in their own food to a place that sold food.
But on the other hand - the price of soft drinks - my usual will be a pint of lime and soda. The cheapest I get it (pub price) is 20p - "Just for the lime - soda water from the mixer tap is free" - then other places: not even city centre/posh pubs charge up to £3.60. for the same - how CAN they justify that? Now that sort of extortion would tempt you to BYOB!