I think I can help you here.
I also went to Alaska earlier this year with my father and we kept 20kg of fish each which we had filleted, vacuum packed and frozen (we subsequently stored them in the freezer facility at ANC and then at SEA airports).
The staff at the storage facility in SEA told me that airlines would only accept the fish if they were sealed in waxed boxes, however, when I asked at BA check in they told me that as long as it didn't leak and was securely packed (we used insulated fabric zip up freezer bags) it was fine. They were extremely helpful. I can also confirm that you have a 20kg allowance per person from the point of view of UK customs.
The fish arrived from seattle still frozen and we have been enjoying delicious salmon, halibut and rockfish since. Well worth the hassle of doing!