In Sainsbury's at the moment gluten free pasta costs the same as normal Sainsbury's pasta (75p for 500g) – prices likely to change between now and next year though.
With pasta: pesto (you can get vegan stuff, or make your own (essentially just blend a load of stuff together)), tinned tomatoes (add seasoning, veg etc.), garlic and olive oil, bolognese (tomatoes, lentils/soya mince/other vegan mince, onion, garlic, carrots mushrooms celery etc. (whatever you like)),
https://www.bbcgoodfood.com/recipes/collection/vegan-pasta-recipes. Marmite (not gf, there's possibly a gf version?), houmous, lemon and courgette, possibilities are endless.
Curry: lentils or chickpeas, a bunch of spices, maybe coconut milk and/or tinned tomatoes, maybe a couple of veg e.g. spinach, cauliflower, aubergine, butternut squash. Serve with rice.
Chilli: basically same as bolognese, but add kidney beans, also some spices (like chilli and cumin), bit of lime juice at the end if you fancy it. Have with rice, maybe sweetcorn, avocado if you have the funds!
Cottage pie: bolognese base but without the tomatoes, add some peas. Top with mashed potato (microwave a pierced baking potato until soft, mash it in a bowl with a fork with some (non-dairy) milk, butter/oil, bit of salt and pepper. Oven for a bit.
Stir fry: cook some noodles (you can get rice noodles). In a big pan/wok, fry: tofu (or other protein) and whatever veg you fancy, stick in some soy sauce, and maybe some ginger/chilli/garlic/sweet chilli sauce/peanut butter.
Chickpea tagine-y stew-y thing: chickpeas, tinned tomatoes, some veg like carrots, dried apricots/prunes/sultanas if you fancy. Probably have with rice or gf bread or some gf couscous if that exists, or quinoa.
Jacket potato: add literally anything – baked beans, preferably some form of cheese (if you can afford some vegan stuff, or maybe some nutritional yeast but it doesn't have the same gooeyness), chilli (leftover from above) for example.
Rice: risotto, spicy rice, fried rice – stick some seasoning in your rice as it's cooking, add whatever veg/protein/other ingredients to make it into your desired dish.
Bowl food ("Buddha bowls" perhaps?): rice, tofu or other protein, veg/salady veg, some sort of dressing/sauce, maybe some seeds or something for some crunch.
Soup + some bread = great warming lunch in winter. Plus some leftover soup could double as some sauce for some pasta (depends how rogue you're feeling)?
Potato wedges, polenta, mashed potato, toast – with whatever. Oats.
Lidl and Aldi are generally pretty good; also have a look in other supermarkets (like Sainsbury's and Tesco) as they often have products that are "price matched" – and take advantage of loyalty cards for discounts. Batch cooking saves a bit of time. Bulk buy, frozen veg (I always have peas, spinach, cauliflower) is great, freeze bread as well, soya milk's generally the cheapest milk, I like to buy dried pulses as they work out cheaper than tinned stuff, little bottles of lemon/lime juice (35p in Lidl last time I checked), peanut butter (£3.99 for 1kg in Lidl and Aldi) (my favourite hehe

).
Basically: carb + protein + veg/fruit, bit of fat; try to have a reasonably varied diet and enough vitamins and minerals (especially calcium, iron, iodine, selenium, vitamin D, vitamin B12). And make sure you're eating enough overall!
