What food from Niue can I cook in the UK?
You've asked an extremely broad question, and no indication of what kind(s) of fish you have available, or want to use.  Fish is a staple in the Niue Islands, and the seas offer up an abundance of such choices such as mahi mahi (dolphinfish or dorado), ahi (tuna), and ono (barracuda), coconut crab, red bass, sailfish. You can substitute any firm-fleshed white fish: cod, bream, hake, monkfish, sea bass, sole, swordfish, turbot. In truth, I would go to the fishmonger and get the freshest, firm-fleshed variety available (and the most sustainable). If what's available is frozen, go with that. Then make Fai Kai, massively simple, fish baked in an umu, an earthen oven, and requiring only two ingredients, fish and coconut cream. I checked and Tesco stocks the latter. Place the fish in a baking dish, pour over the coconut cream, cover the dish with foil, and let it marinate for about 30 minutes. As I'm pretty certain that you don't have an umu, or any plans to dig one, bake in a pre-heated 180 C (gas mark 4/350 F) oven until done, about 30 minutes.