@camlok,
Pretty much, pie and chips, sausage and chips, spam fritter and chips, fishcake and chips. In Northumberland you can get haggis and chips.
The main fish affected by overfishing was cod, and some years ago there was a campaign to get people to cut down on cod consumption, one alternative trialled was coley, although cod is now back on the menu. Even then haddock and plaice were unaffected.
The main reason for decline is increased competition. Fish and chips used to be the only takeaway food available. Now there's a huge amount of variety. I remember going on a coach trip to London and being told how we'd visit McDonalds for a treat seeing as there were only about 2 or 3 branches in Britain and they were all in London. Now they're bloody everywhere. Pizza was unheard of until the 80s, now they're bloody everywhere too.