of course it's doable, I lived off £25 a fornight this year and I was never short of food or ideas. You just need to shop around properly - go for discounted brands or offers and see what works out cheaper, buy things that you will use a lot of in bulk rather than smaller packets, buy dried things like pulses and beans and most importantly, learn how to cook. That will save you infinite amounts of money. Buy stuff that you can freeze as well and make stuff that if you make too much of it you can freeze and eat another day, don't buy things that won't get used up cos it really is a waste. Pasta sauce jars are alright or you can make your own sauce dead easy as well.
On the other hand morrison's do £2 for 2 pizzas and stuff like that, but then you'll end up getting fat.