We had been tipped off that it was possible to buy good seafood in the old port office at Port Penrhyn, the surviving and attractively ramshackle old port to the north of Penrhyn Castle, much of the port rebuilt in 1820 before the castle itself. You can indeed, but only from Friday to Sunday. It provides a pretty impressive array of fresh fish, as you can see from their blackboard:-
| Charles | Saumarez | Smith | 
Only at weekends! British fishermen do often seem rather unenterprising. In May I was in Bridlington, which apparently lands lots of lobster (or did before Brexit). I spent three days trying to find some for sale, wandering round the town and Googling enthusiastically. I eventually discovered someone selling lovely shellfish from the garage of a suburban house.