You could get tinned fruit? Keeps for ages and actually could be used in warm recipes just now as well as cold/to top breakfast/yogurt etc
Also dried fruit - tends to come in small packs and while yes it's sugary it's also good for fibre and nutrients especially iron
Would you be able to buy vitamin supplements with the voucher? Controversial idea maybe but Tesco has a "3 for" on at the moment on these I just got some myself - vit c, vit d and b complex (I suffer badly with mh issues and I'm worse in winter so hoping to mitigate at least slightly plus ward off winter illness)
Cous cous - a big good value bag! Usually best found in the "world foods" section. If you have somewhere to store? Not the ready flavoured stuff as that's more expensive and limits what you can have it with. So quick and easy to make and a great alternative to rice and noodles
In terms of cheap treats check out the kids sweets section - 3 for £1 on things like dolly mixtures, jelly beans, cola bottles etc sometimes a little nostalgia really helps
On the herbs and spices side, again "world foods" aisle is usually cheapest, also maybe get "blends" or "mixes" rather than lots of individual ones? Eg Cajun spice mix, curry mix, chilli mix, Italian seasoning ?
Remember also to get stock cubes! Can make a huge difference!
Other than that it depends on what you already have and your preferences.
I'm vegetarian and while you're not you may find certain veggie food is ok for you and is usually much cheaper than meat versions
Eg tinned veggie ravioli is half the price of meat, tinned lentils can be used instead of mince more cheaply in many dishes, veggie sausages are cheaper than meat depending on brand, I like the Linda McCartney original frozen, they're soy based I think and quite meaty in texture and well seasoned, they go great with baked beans. Well you get the general idea I'm sure