The Victorian age is back in the UK and seemingly not only by wealth distribution (that has already been achieved a few years ago). The NeoCons would say we're half way there (on the journey back to times before the Age of Reason).
Even still, I myself that groceries I eat are super cheap compare to food I throw away and restaurant meals. (And I avoid packaged meals and name brand cereal, which are near restaurant priced)
Every now and then I remember that the vast majority of people don't or can't think about food costs, and shudder. Paying $2 for a bottle of water with a scoop of sugar poured in. $5 for a box of Kellogs instead of $3 for generic. $4 for the frozen brocolli with fake cheese sprinkled top instead of $2 for plain. Etc
Now, we consciously look at prices before putting items into the cart, and like you said going for store-brand and avoiding packaged food. (On non-packaged food: I am continually astounded at the ridiculous amount of fruit and vegetables you can buy with just a few dollars.)
In this way, we save literally hundreds of dollars per month. It is downright incredible!
There is one absolute best way to spend less on groceries, and it is something that everyone can do, regardless of where you live, what diet you're on, or how much money you have.
What's the secret? When you're shopping, write down the cost of every item before putting it in your cart.
You will be amazed at how your food choices will change. For example, once I started doing this, I shifted to buying fruit and vegetables that were in season (and thus less expensive), rather than just buying things randomly and oblivious to the cost. The same goes for choices of meats, fish, and even non-food household items like soaps, detergents, toiletries, etc.
This is by far the best method of spending less on groceries that can apply to everyone. If you've never done it, I challenge you to try it just once. And if you have any suggestions on how to make this strategy even more effective, please let me know!