I haven't been through Islington in a while, but I do recall a very nice Fish and Chip shop on Stoke Newington High Street called Tina's Fish Bar. Also, near Highbury and Islington stn, there is a place called the Trawlerman that I also thought was rather good. However, their portions were rather small. I cannot (or will not) eat an entire regular portion of Fish and Chips anyway, so it wasn't a big deal for me.