If you have easy access (like on foot, perhaps) to an airport or port from which people travel to Ireland or the european continent then you might be able to do a euro-to-sterling swap with those who have no use for the sterling (foreigners) or need coins for telephones or supermarket trolleys (Brits). Bus stations where continental buses leave from might also be of use. You could also offere the coins via a supermarket advert board. It all depends on how desperate you are as to how hard you want to try.