I know this is old but for anyone else wondering about costs:
Breakfast; eggs, bacon, cheese in any way or combo (muffin, omelette, boiled, scrambled)
Lunch: Tuna or chicken salad with mayo. Feta salad with olives, or ham wrap made using iceberg lettuce. broccoli and stilton soup or cauliflower and bacon soup.
Snacks: cheese cubes, olives, nuts, sugar free jelly with whipped double cream. 85% green and black dark chocolate bar is £2 ifyou can ration yourself.
inner: frozen fish cooked in parcels with butter, herbs and lemon or panfried with broccoli, cabbage or cauliflower, sausages with cauliflower mash, mince meat or burgers (check carbs or homemade low carb) with salad or warm veg.
Drink: coffee or tea with double cream, stevia if you must,
Shop at lidl and buy own brand.
Set your budget and then see what you can get with and make a decision whether it's doable for you or not. It would be useful to compare a previous shopping receipt as though pasta is cheaper than chicken, in reality you may find you also buy crisps, biscuits, fruit (which are expensive), ice cream, pizza..etc.