There are a few landside restaurants and pubs at Heathrow - there's a 'Spoons at T2 departures, for example. They'll certainly do fish and chips, but it'll certainly be a pre-prepared version. Very cheap, passable at best.

The Hot Spot fish bar in Harlington gets a good name (never been there personally). The 140 bus goes there from Heathrow Central, but you can go there for free if you get off a stop or so early.