Popped in on a very wet day for sandwiches & coffe...

Popped in on a very wet day for sandwiches & coffee.
When it came to pay we were told our Northern Irish sterling was not accepted at this store & even the post office would not take it according to the gentleman behind the till.
Would like costa to confirm that they do not accept Northern Irish sterling please.
Staff could do with some customer training as this guy was down right ignorant.