My mum refuses to buy healthy food, how can I stop this?

Hiya. Your mother seems to be in denial, and most likely anything you try to say or do will push her more into that. In many situations, even when the logic is sound, denial will be strong for one purpose only: inability to admit any error. The more one is pushed, the more elaborate the denial dance will become. Including putting the blame on you, argument responses which shift the topic, and other 'logical fallacy' and 'cognitive dissonance' tricks used by people in denial.

But one main thing cannot be ignored: Without or without your health concern... your desires, goals and needs with regards to food is being blatantly ignored by your mother. Add to it the health concerns, she is adding another layer of disregard on it. Tossing out your insight as to healthy eating, she is also suppressing who you are as a person.... namely an intelligent young human. In fact, a 17 year old who wants to eat healthy... well that is pretty rare, and that rareness should be nurtured. I'm a tad disappointed in your father too, because he needs to stay on top of this concern and push his wife more.

Because of the above, and until you move out, I think you have no choice but to go into survival mode. If you cannot afford to buy and keep your own stash of food, default to the next best thing: eat as much raw veggies and fruit as you can, and reduce the other crap given to you. Raw, to get the most nutrients. And water instead of her sodas. I realize that she doesn't seem to buy many veggies so you will have to figure out a scheme. Perhaps bugging her for a few more veggies, and no other change, might work. You can still eat her meals, just munch on veggies in your room. Other options... trade with friends, find a caring relative who could help donate a secret veggie/fruit stash, volunteer at an organic growers in exchange for some food, ask restaurants for good but old veggies, job then buy your own, dunno. Or somehow get your hands on some good supplements.

Point is.... I don't think you can change her. And it appears Dad might be living in mental haze, perhaps due to the food. I think you need to go secret agent on them and find a way to get healthy stuff in you, to counter balance the crap food that you cannot avoid.

I think you should sit down and have a chat with your dad, explain what's going on. Seems like your mum has a problem.

You cook the meals you say? Marinade her next steak in piss, sprinkle some nail clippings in for texture