Cheese roll-ups (with butter, ham, whatever), you could make egg muffins ahead of time (your basic omelette but in a muffin tray: bacon, chives, smoked salmon, put in whatever you want), and have them cold, or nuked. Cold cuts, high meat content sausages (made at home, nuked at the office), strawberries with (clotted) cream, pork scratchings, boiled eggs... Here in the Netherlands most supermarkets have a tapas counter with little containers with all sorts of stuff, cheeses, prawns, sausages, meatballs etc... I don't know if there's something available like that there?