I don't consider a charge for club soda to be unreasonable. But as kipper and wpxstar mention, a restaurant often has to make the choice between being right (charging for something they are reasonably entitled to charge for) and making money (by leaving the customer satisfied and eager to return). To ignore this is to step over pounds to pick up pennies.

OP, you might have had a better outcome if you had explained to the server that you didn't realize that the club soda was chargeable. A well-run restaurant would have cheerfully removed the charge and left you with a positive impression of your experience at the restaurant.