Agree with a lot of the suggestions above.
Obviously it will depend what you have in your cupboards / freezer etc, but I'd get value pack of mince, whole chicken, pasta, rice, value sardines, potatoes, onions, garlic, carrots, whatever greens are cheapest, frozen peas, big block of value cheese, family pack mushrooms, red lentils, tinned tomatoes, maybe baked beans, eggs, wraps, bacon lardons. Lettuce / cucumber. Porridge or big box of own brand weetabix and some bananas. Maybe apples if you can. I'd try to go to Aldi or Lidl as you'll get more for your money. If you can get a Lidl bargain fruit & veg box, even better (think they are £2.49).
Big pack of mince should stretch to a few meals - make a bolognese type sauce with lots of veg, eat with pasta, top with mash, add chilli / beans, leftovers with a jacket spud.
Roast chicken - cook extra potatoes & veg, use them the next day for a hash / bubble & squeak type thing with baked beans / eggs / bacon bits, anything you have in the cupboard / freezer. leftover chicken use in fried rice, pasta, soups, sandwiches.
Quesadillas - pack of cheap wraps & cheese with any odd bits of veg / beans etc you have lying around, few spices if you have them.
Lentil dhal, veg curry, cheap & filling.
Pasta - spaghetti with a couple of tins of value sardines in tomato sauce, chilli flakes, garlic, capers, olives if you have them. Or just a tomato sauce - onion, garlic, tomatoes and any leftover veg. Roasted peppers - big jar for not that much at Lidl and blitzed with tomatoes is a really nice sauce.
Soup & bread / sandwich for lunch if DP has a food flask or access to microwave at work.
It's tough but doable, and will be harder if you don't have any basics in already (oil, flour, spices, rice, pasta etc) as that would eat into the budget. Personally I'd scrap the Halloween treats but if you can't, then B&M and Home Bargains and the like often have big bags of sweets for not too much. Good luck :)