In reply to mutt:
In England and Wales, by law restaurants and cafés are legally obliged to serve tap water free. But since the law is full of contradictions, they can charge you whatever they want to use the cup, or even refuse to let you use their cups. So refusing water may be illegal but requiring you to provide your own receptacle is not. Unless they issue disposable cups, what else can they do?
Pre covid, 'd have grumbled about the £13 and cups, but so many walls are struggling not to go bust. I suggest you give them the benefit of the doubt and take a bottle - or go and buy something from their cafe and help keep them open!