Octopus-shaped sausages and panda-shaped onigiri are classics in Japanese school bentos. But you may be too old for such childishness.
I'd suggest having two boxes, or a box with removable compartments, so that you can carry both food to heat in the microwave, and salads and such to eat cold.
edit: I think "deep-fried mars bar" is more of a joke/novelty item than a real thing that people eat. It's a joke about people from Glasgow being unhealthy.