mysupermarket.com is great, shows all the offers in the stores nearest you. Can either do an Internet shop or make lists to take shopping. You can add your favourites from the supermarket websites and as you plonk stuff in the basket it shows the running total in all available near-you supermarkets which deliver. Then if you realise ASDA would be cheaper than Tesco forthe trolley you're filling, you can switch supermarket.
www.madaboutbargains.co.uk can be useful too.
I live in a town of 50,000 people which has Tesco, Tesco Metro (another opening soon),Co-op, Costcutters x2, ASDA, Sainsbury's x2,Morrisons, Lidl, 2x Polish shops, 2x bakeries and 3x greengrocers. Ten minutes' drive to next town in one direction there is Aldi, Netto, Somerfield (soon to be Co-op) and fifteen minutes drive to next town in other direction there is Aldi and Waitrose. All three towns have a Holland and Barrett.
With that much competition for our food money, we get bombarded with special offer leaflets from supermarkets. What's worrying, is that many supermarkets seem to restrict special offers to crisps and chocolate and carbs and rarely discount healthier stuff at all !