One thing that Tesco used to sell was Pizza Toast. That was just a sauce with herbs, onions and some other little bits in. Use it with cheese on toast and it was an awesome snack. Not seen the product on the shelves in a few years though.
Not sure if these count as simple food or not, but proper Japanese Noodles, with a bit of sauce (whichever you personally prefer: HP, Ketchup, Soy, other) are nice and relatively simple for a snack.