My mother used to make coleslaw, both when she was living on her own on in her 20s, and later on when we were young. On her own, she would finely slice a cabbage, some celery and a pepper, and grate a carrot or two. Dress it with oil and vinegar (+ salt, pepper, tiny bit of sugar, and herbs if you have them) - it will keep a week in a box in the fridge no problem. If you prefer it with mayonaisse just add it as you want it. Then omelette + coleslaw makes a meal. Buy value potatoes, bake several together, reheat one at a time. (or, microwave most of the way and finish in the oven). Look in the supermarket for reduced veg but only choose something you know you will use or that is massively reduced enough to justify giving it a try.
Look at frozen fish and meat - you will have to manage your budget carefully but I think I have seen 1 kg chicken portions at around £3.50-£4, similarly you can get frozen fish portions. I often speed up the cooking of chicken by microwaving it and then finishing it in the oven, even though the packaging often says not suitable for microwave cooking. If you can gradually stock your freezer with cheaper meat and fish options, then you can vary your meals through the week quite easily even if the veg stay the same for a bit.
Another cheap meal trick my mother used a lot in my childhood was to pre-cook mince and then use it in a casserole or stir-fry. Brown the mince in a little oil then add a bit of water and simmer for ? 1 hour or so. Keep it in the fridge. For a meal, chop onion / carrot / cabbage or similar / anything else you feel like, fry in a pan, then add a spoon or two of the cooked mince along with any seasonings. Vary by adding a tin of chopped tomatoes. Serve with rice, potato, pasta - or as is if you are low-carbing :)
Good luck figuring out the right way for you to make the budget stretch.