To whomever pays, it's a global marked. If wild salmon from the pacific can be sold in Europe, why can't fish from the UK anywhere? As for theorizing about lower prices, are fisheries very elastic? Is demand very elastic? If not, how are prices going to fall? If access to those waters was not an advantage the EU would not be so eager to get it. Remember that in a negotiation the first trick is to claim that the thing negotiated is worthless. Actions will speak louder.