You will definitely find better options at the station than onboard.

M&S (Marks & Spencer) is the best for sandwiches, salads, cold drinks and snacks. (If you get a salad, don't forget to pick up free wooden cutlery at the exit.) Their own-brand chocolate is excellent, too, mostly made in Switzerland - I particularly love their version of Toblerone.

Pret also good for sandwiches and pastries, though a little expensive. And there are many coffee outlets - Pret or Caffe Nero probably the best.