It's nigh on impossible to shop in a way that finds all the best prices and beats the supermarkets -- a lot of their game is befuddling the consumer. So, prices may stay the same but packages shrink in size. Or they'll have a value range but the number of products available will vary or decrease, or they'll suddenly rebrand it so you can't find it anymore.
My mum was a very good household manager and she taught me a couple of things which stand me in good stead:
1. Anything that's on display at the end of the aisle or near the checkout are probably not good value. The store is literally flooding you with those products.
2. Always look at the products stocked either low or high on the shelf because everything that's at eye level is likely the one the shop wants you to buy.
3. Bulk buy if you can, but only if you will use the stuff before it's out of date and you have space for it.
4. Don't look at the price tag but the price by weight. Because that helps you figure out what's lowest price, even though the packages of different brands might also be different sizes.
Look for pasta, for eg, and Barilla will be £0.60/100g and Sainsbury's Own will be £0.40/100g.
It's hard work, initially, but you get used to it and it works both in the shop and online.