Butter? It sits in butter mountains for x years, so it's not going to magically go bad on the best before date. We often eat yoghurts a couple of weeks beyond sell-by, and vegetables/eggs (even meat at a pinch, though probably not fish) we do on a common sense basis. (Though with irradiated veg, sometimes something can look perfectly fine but isn't).
So I'm baffled that the Somerfield near $work sells salad that is borderline disgusting. Salad should be crisp, not limp, soggy, and blackened. Often even the in-date stuff looks bad, and by the time they get around to putting a label on it there's no way I'm going to buy it to eat raw certainly, though the watercress/spinach/rocket packs could maybe make a soup.