Greenshadows, yes. The OP here drank tap wáter, but the number of times I have been out with friends who are either 'not drinking' or are pregnant, sit there and say 'I'll 'just' have an Orange Juice, or I'll 'just' have a coke' and wait for me to go to the bar. It costs MORE than the pint I am having.
In an expensive gastro pub in Exeter the other day I ordered a pint, offered my friend a drink and she said 'I'll just have a pot of tea' clearly not intending to pay as it was tea and I was having a pint. The tea was more.
Agree with znaika
Here in Spain, (where I normally live) a beer is a euro, a coke is 1,50.
Aside from tap wáter, don't asume your mineral wáter, coffee etc, is cheaper than the split bottle of wine.
I drink (alot!) but never have the pudding, never have the coffee, I resent paying for those who do. Coffee in restaurants in the UK is generally bad and a complete rip off, yet everyone orders it and I sit cringing at the extra X amount that is going to be added to the bill for a really horrible coffee.
However OP, my little rant aside! YADNBU, just learn from this and say at he outset.