I've taken frozen vacuum packed salmon and tuna to England and Australia before. I just wrapped it up in towels (or the best insulator in your bag) before going to the airport, and put it in my checked luggage. It was fine by the time we got there, still frozen. I declared it in Australia, as they get bent right out of shape with protecting their agricultural industry, the Customs guy took a look at it and said it was fine. Fresh fish (or any unfrozen product) is different, that they'll generally confiscate.