Do you have a freezer?
A freezer is a godsend. Get loads of microwaveable plastic tubs from Poundland, and spend a day cooking massive meals - curry is a good one, or ratatouille, or Shepherds' Pie, or anything you can literally just microwave.
You have to keep an eye on the health and safety aspect (I've never poisoned myself yet), but they can be pretty nutritious, and you can plan weeks in advance without thinking about it. Supplement with Porridge / cereal for breakfast, and sandwiches or soup (soup is good for nutrition) for lunch, and fruit during the day, and you have a pretty health, easy diet.