Indeed. There was the man in the US who lost weight on the Twinkie diet, eating nothing but cheap, nasty prepackaged cake. You'd be hungry and obviously it's very unhealthy, but as long as you burn fewer calories than you eat, you'll lose weight.
There's also the man who's eaten nothing but 3 Big Macs a day for decades and he's thin, because that's only about 1600 calories a day.
A slow cooker is a great tool to eating on a budget, because you can make all manner of soups, stews, curries etc with lots of cheap veg and pulses, using little fuel.
See if you can manage to get hold of some cheap, basic spices - own brand jars or Asian branded packets from most supermarkets are about 50 pence each so if you could get a few of these, it would make a huge difference to the meals you could make. Jack Monroe is excellent for cheap meals that don't use endless ingredients.
Depending on what you like to eat, chilli powder, cumin, coriander, smoked paprika, tumeric, garam masala and curry powder are likely to be what would be most versatile. If you can also get frozen garlic and ginger, some supermarkets sell big packs of frozen cubes for about a pound each - cheaper than buying fresh.