Get some basic ingredients in. Things like potatoes, tinned tuna, tinned sweetcorn, tinned tomatoes, bread, milk, eggs, cheese, mince, pasta, lentils.
Potatoes. If you get a bag of medium sized ones, you can use them for allsorts, rather than buying a more expensive pack of 4 baking ones. Buying loose is usually cheaper (for everything!)
Tinned tuna, cottage cheese on potatoes is good. Maybe some sweetcorn too.
Mince. If you do shepherds pie or spag bol, do double the quantity to help save wastage. You can either have some for lunch/dinner the next day or put the other portion in the freezer for a home made ready meal.
also when cooking the above, you could replace some of the mince with lentils. Very healthy, much cheaper and a similar taste as long as you don't overdo it.
Eggs are cheap. Make a good breakfast

Pasta is very cheap and versatile. Tomato based sauces etc.
I love cauliflower cheese. Make my own white sauce as it's cheaper (and nicer). Dont' waste money buying cheese slices etc. Slice your own. Cheese isn't cheap, but again very versatile.
If you buy really strong cheese you will need less of it and save on the pennies.
Always look for bargains. Go to shops at the end of the day, consult shopping websites such as
Supermarket Special Offers Tesco Asda Sainsbury's - Free Search and Save money on your shopping UK Prices
Chicken is quite cheap too. Buying cheap chicken is a bit of a ethical concerns, but when money is short, then need must............
Don't buy portions though. Get the whole thing, and use all of it if possible. Add to pasta, make a sauce for it etc, put it in sandwiches.
Avoid buying ready made salads. They're really expensive and often don't last.
I like iceburg lettuce, and find it much better to buy the whole lettuce than a bag. It keeps for so much longer and I can just 'cut a slice' when needed.
I could go on for ages. Hope that gives you some ideas.