there isnt a limit on tinned tuna but there is on fresh tuna. Only women trying to get pregnant or who are pregnant need to limit the amount of tinned tuna they eat.
From the food standards agency:
People can eat as much tinned tuna as they like, except women who are pregnant or are planning to get pregnant in the next year. These women should eat no more than four medium-size cans of tuna a week (with a drained weight of about 140g per can), or two fresh tuna steaks (weighing about 140g when cooked, or 170g raw