I read somewhere a while ago that in the UK if you're a restaurant / a bar ... if someone comes in and asks for a glass of water you legally have to give them one for free (and I think to their horse as well) ... I believe it goes back to old law that someone could of been travelling by horse on a long journey and need water just to survive.

I've never put it to the test though ... as others have said I've gone in and bought something like an energy bar / sweets and asked if there's any chance of some tap water as well.