I used to blow £70 quid on food which would last me 3 weeks. But now I know that isn't needed and a week's shopping is easy £20-30 if you eat right.
But for me, I'm a foody so I find i really difficult to eat supermarket-branded stuff like bread, pasta, beans, cereal cause most of their carbs literally taste like cardboard and it really isn't nice to eat horrible tasting food.
One piece of advice definitely shop around! For example, I found it incredibly cheaper if I were to get all my frozen stuff from Iceland (don't buy your meat there) and then buy my everyday stuff from Tesco. Iceland always have some great deals on including branded laundry stuff! If you can; do a day's of shopping every week. Go to Tesco in afternoon then Iceland in evening. First years are always keen to go shopping together and walk it into town and etc. But then find it a lot of effort and cba and decide to do Tesco online. What I did was remembered whilst it was easier, I was definitely getting fatter. I think walking into town whether it's a 15 mins or 45 mins walk is a good way to keep relatively healthy with exercise (especially if you pile on pounds from just eating like I do).
Oh and another thing don't get ready-made meals. They are a total rip off. It's something stupid like 3 for £10 or something. No mate. If you get individual ingredients it's much cheaper and usually you can make those meals in bigger quantities. I would do this cause it saves so much more money and generally healthier and you will learn how to cook. It's much better as well.
If you get on well with your flatmates try and do a weekly cook where you guys chip in a fiver and help cook. It's a great way to bond. Albeit I never did this at my first uni, but we did do this for our Christmas meal and it was the best thing ever!
But yeah, shop around and cook in greater quantities.