A good start would be a higher rate of vat on fast food, takeaways, crisps, chocolate bars, fizzy drinks, pies etc. Plus remove zero rate vat on cakes and other high sugar, high fat processed foods. Only keep zero rate vat on basic foodstuffs such as fruit and veg, raw meat and fish, bread, milk and cheese, etc - basically perishables only, anything that needs to be stored in fridges or at lower temperatures is zero rated. Make healthy options cheaper than unhealthy.
Of course, the public won’t stand for that. Just look at the pasty tax fiasco where a perfectly reasonable suggestion of vat on Cooling pasties was suggested. Had to be overturned because of the backlash.
Anyway, when food banks are happily handing out high fat, high sugar, heavily processed tins and packets full of additives and preservatives, any vat increase won’t change behaviour will it, because the food bank users aren’t paying, so maybe better to ban food banks from handing out the crap heavily processed stuff.