Alternative fish for Fish and Chips
I have often heard that some restaurants use Atlantic pollock for fish and chips. Your question piqued my interest and I decided to look around a bit.
 From wikipedia ,

In Britain and Ireland, cod and haddock appear most commonly as the fish used for fish and chips,[31] but vendors also sell many other kinds of fish, especially other white fish, such as pollock or coley, plaice, skate, and ray (particularly popular in Ireland); and huss or rock salmon (a term covering several species of dogfish and similar fish).
In the United States, the type of fish used depends on availability in a given region. Some common types are cod, halibut, flounder, tilapia or, in New England, Atlantic cod or haddock. Salmon is growing common on the West Coast, while freshwater catfish is most frequently used in the Southeast. In Canada, pollock, haddock, and halibut are popular choices, alongside cod.

From my experience, pollock and halibut (respectively) would be closer to the the flavor of cod and haddock than some of the others.