Historically, most of Britain was too far from the coast for fresh sea fish to be eaten. Instead people ate smoked or salted fish. Although kippers, Arbroath Smokies, smoked cod, smoked haddock and the like were popular a generation or two ago, the decline in home cooking and the rise of "international" fast food and pre-prepared meals has certainly meant a decline in their consumption. Coastal and fishing villages always consumed a lot of seafood; fish, crustaceans and bivalves etc, but usually very simply cooked, so no "great" cuisine developed as it did in France, for example. Fish was also seen as "poor man's" food, preserved in some form inland or fresh at the coast, it wasn't seen as being food for the rich, unless it was sole or salmon, in which case it would be cooked in a French style.

Thanks for asking this question, it's something I've always wondered. One thing that should be noted is that some seafoods that used to be popular in Britain are now unfashionable, for example jellied eels, whelks, and cockles.

"Fish and chips" is certainly part of the "signature cuisine" of the UK.

British & Irish Isles.