One of the most important things is to only make one meal for everyone. Anyone that won't eat it gets to have a bowl of cereal or a slice of toast, but nothing else. Making more than one meal really pushes up the cost.
Another thing to do is to use frozen instead of fresh veg, for instance you can get 8 frozen half sized corn on the cob for about £1, so 1/2 the price you just paid.
Make your own potatoes, takes a bit longer, but is much cheaper, same for sauces/pizzas and the like. You can make a basic sweet and sour sauce with tomato ketchup, vinegar, sugar and soy sauce for pennies. for instance.
When I meal plan I have a price in mind for meals, usually between £2 and £3 for the lot. Sometimes it's a bit more, but there are usually leftovers for lunch or whatever, and sometimes it's less. So it all balances out.
I tend to plan around one ingredient, perhaps the carbs, and spread those meals out over the month, so that we eat pasta 2ce a week instead of for 2 weeks on the trot. I only cook meat once a week, maximum 2ce, unless I do a joint which I portion out to last a few meals. A chicken would make, for example - creamy chicken and veg pie, chicken pasta bake, chicken soup and spicy chicken and tomato tortilla bake. A ham joint would make - ham and veg pie, pea and ham soup, savoury rice, cheese and ham pasties and creamed ham jacket potatoes.
I tend not to buy meat that is "single use", chops and the like, because it's too expensive. Fish is either tinned or frozen and only eaten once a week at most.
I get paid monthly, so I plan and shop monthly. When the fresh stuff is gone, it's gone, we have tinned/dried fruit and frozen veg (except the guinea pigs, they eat whatever I can get cheap). I buy things like crisps, biscuits etc. once a month and don't replace. When they are gone, I make pancakes/banana bread/yoghurt loaf/plain muffins for snacks. If they eat all the cereal, then it's toast for breakfast. We drink squash or fruit tea, except Dh who has a coffee habit. I buy a few bottles of flavoured water and sparkling juice once a month only.
Eat seasonally, no strawberries in winter for instance, buy plain fruit and veg (apples, bananas, oranges, not blueberries and mangoes, carrots and cabbage not asparagus and squash.)
You need to make a list of the kind of meals your family like, Then look at what you have in to see what you can make. Pick a few more meals and decide what you need to buy. Look at nutrition over the course of the week rather than individual days and take it from there.