I've been digging around some great price comparison sites for saving money on food and I would recommend you check these UK supermarket comparison sites out.
Money saving supermarket provides guides, offers and best buys for food prices, shopping, credit cards, broadband, energy prices, banking, savings, loans, and voucher codes.
This site looks at supermarket shopping online and claims to save an average of £17 per shop!
They say "Everyday we collect over 50,000 items from Tesco, Sainsbury's and Asda and you can easily and quickly search them. If you register you can build your own shopping list and see at a glance which supermarket would be the cheapest.You can compare over 10,000 items to see who has the best buys. Our sister site is updated daily and has over 5,000 special offers. "