My main tip is bulk cooking, I find most ingredients are too much for one meal and rather share it across multiple meals, if you make 2 3 meals store them in good quality plastic containers in the fridge and then have a singular meal you can quite easily get main meals down to £1-2 a portion but still afford lots of nutrition.
Other advice is frozen is best, morssions has a decent frozen range I have found and is well priced although inflation has caused it to be borderline too much for my liking. Aldi, Lidl, and Tesco also have decent affordable frozen ranges although I find the variety not great and not consistent stocks between stores. Also, CANNNS, with frozen and cans you do not need to cut anything
I get by with just spending £20 or more often under a week on food although I take bulk making to the extreme with making a meal for 6 days with ingredients costing around £10 for all days. For instance this week I made a curry with sweet potato, peppers, chopped tomatoes, coconut milk, a mixture of species green lentils, chickpeas, canned potatoes, peas white and red onion, spinach and herbs, then just made rice for two days saving half for the next day and repeat until the meals are over, as it's nutritious and tasty I don’t mind having it every day, then I'll have something else the next week and probably not do the same for a month and rotate. As I said earlier the only thing that was not frozen, dry, or canned were the sweet potato in which I find fresh is better than frozen.
Meat wise frozen is also amazing, I do not eat a lot of meat but when I do it is frozen, mainly bacon or sausages from Lidl, for about £2 you can get a decent amount. Bacon and sausages are good as they have good visual gauges of when it is done if you are not a confident cook and worried about food poisoning, key thing is just made sure nothing raw contaminates anything (mainly just thawed out bacon and its juices as you can’t cooks it frozen, sausages can be though)
My thought process is thinking what cuisine I want, choose a carb then just buy things to bulk up until I reach the budget with variety being key as variety brings different nutrition and flavour, then also think of different ways you can cook an item as different things will introduce different textures and flavours. Then also stock up on spices, again Morrisons is amazing for this if you go to the international food aisle you will find common spices like coriander, cumin, turmeric etc under the name of Rajah, they are extremely excellent value compared to the jarred stuff. Aldi and Lidl also have increasing stocks of spices, a favourite of mine is smoked paprika which I will use in a lot of things as a smoky flavour is nice and paprika works well with sweet things like tomatoes.
I do not have recipes as don’t follow them, but the trick is to just experiment just because you are on a budget does not; mean you cannot try new things and flavour combinations. Also sticking to veggie meals means you do not have to worry about serious food poisoning.
Eggs are good too, also cheese, blocked cheese can last a lot of meals, never buy grated, another favourite of mine is quesadillas