Get thee to Money Saving Expert forums! - Old Style board. Then check out the Weekly Meal Plans and Grocery Challenges threads.
The cheapest way? Meal plan, meal plan, meal plan! At the start of every month, I make a mega batch of tomato sauce in my slow cooker - 500g mince, 2 tins of tomatoes plus lentils, oats & grated carrot to bulk (the slow cooker reduces all this to lovely sauce and you will NEVER know it's in there). Out of this, I generally get 4x meals (for 2 people) which can be made into: spag bol, lasagne, chili, etc.
Cooking a whole chicken, like others have said, also works. Basically, the freezer is your friend. If you get sick of chicken, get a turkey leg for £1.50 and roast that...another several meals: curry, casserole, etc.
Buy large blocks of cheese and grate them straight away. If you leave in blocks, you'll be tempted to eat. Stick the grated bags in the freezer and you have instantly ready for cheese sauces, topping for jacket potatoes, top for cottage pie, etc.
I've picked up a zillion ideas from MSE and saved sooo much money. Can give you some samples of my monthly meal plans to give you ideas...though they tend to not be overly glamorous!!