Whilst you can definitely eat healthily and nutritiously on a budget, £30 a month is very very low and practically impossible, even if you’re eating pasta, porridge and toast most days.
I too live alone and if I can eat healthily a week on £30-£35. This is sometimes what I naturally spend on food a week if I don’t eat out or buy any bottles of wine or get hosting food in. Within this amount, I can have Greek yoghurt and fresh fruit with eggs for breakfast, and something like chicken salads, vegetable stews, goulash, crudités and humous, soup and bread, stir fry, chicken and rice, omelettes or sausage casseroles for lunch. I intermittent fast though so only have a big breakfast and lunch and don’t have dinner, but I drink hot drinks liberally through the day. If I was having dinner each day, it would be more like £40-45. Then there are times you need cleaning and kitchen household products and basic toiletries. That will need to be costed for too.
Are you quite young or living alone? That’s the only way I could explain you not understanding how much an ingredient costs, what goes into even a basic meal, the importance of getting proper amounts of fat, protein, fibre, macronutrients in etc. Your comment about a slice or two of pizza at Greggs is perplexing as it’s about 2-3 days of your food budget!