Sandy, if it says fish fillets on the box, it's pollock.
If it says cod fillets, it's cod.
At the moment, Tesco sell the fish fillets for £4 but the cod fillets are on special at £4.50, well worth the extra .50p.
If you shop at Tesco, try their chunky cod fillets. They cost more than Birds Eye but the cod has always been lovely when I've bought them.
They do battered and breaded at the same price.
Make sure it says chunky on the box.
https://www.tesco.com/groceries/en-GB/products/255780854