Keep a log of your shopping, everything you buy. You'll soon see what you're wasting money on, you should be able to feed 3 on £150 a month. The cost of pre-made sauces (pasta, chilli, curry, Chicken Tonight etc) adds up quickly and the little things like the way that people will only buy chicken breast at £5.99 a pop rather than eat the rest of the animal, steak mince, etc etc. There is plenty of very good qulity food out there for next to nothing, you just need to avoid the premium cuts. Make up some huge stews, chillis, spag bols, curry, soup etc all in one go and freeze it all down. A big enough roast makes food for days in leftovers. and you can chuck it in said sauces. Up your vegetable intake and reduce the amount of meat you buy, it's a good habit to get into. Drop things like crisps and fizzy drinks if you normally get them and that will knock a big chunk off your food bill.
Stay out of iceland, it's all frozen junk food that looks cheap but you can make much better quality yourself for less.