The answer to that is largely dependent on each person's eating and shopping preferences.
I go grocery shopping about twice a month, and sometimes in between for miscellaneous stuff as well. I would say it costs about £100-130 for the entire month, depending on where I shop. It's mostly Lidl for me but I also sometimes go to Tesco where the prices tend to be higher. I don't usually eat or drink out a lot, so for any outings, I'd say it would cost around £20-25 each month.
Too Good To Go is a really good app actually, it gives you food at discounted rates from good restaurants in your city. I've seen many Durham students use it and it comes in very handy when you're too tired to cook but want a delicious meal.