I'm sorry, but tuna being farmed in international waters shouldn't be an obstacle. In all EU countries and UK there are quotas of what a fishing vessel can bring into port. So it doesn't matter where the fish was actually caught, if the country ran out of annual quota for tuna for instance, then no fishing vessel can bring any tuna to any port. It's not an insurmountable problem.
Occasionally I find myself in some gastronomy setting were nothing in the menu appeals to me other than the dishes containing tuna and then I eat it, and enjoy it a lot. It happens less than once a year.
(I do actually share your position wrt farmed meat, those farms are infinitely more regulateable than international fishing and I think we should regulate it hard, hard enough to solve big parts of the inherent inefficiency of putting nutrients though animals before they reach a human mouth by price)