Well , you're in luck. About the same day your thread began Mysupermarket launched an app that does exactly this. Free for iphone from App Store but am sure there will be an android on too. V good app, I just shopped in Asda tonight which I never normally do because I found out some of my constant purchases were on offer
The website www.mysupermarket.co.uk works like a bit this it's designed so you put in your whole shopping list and it compares prices, but you can just put in one item, I don't know if they have an app.
Does anyone know if this app exists? I want to type in to my phone Toilet rolls, say and for the app to tell me which supermarket has them cheapest at that time. Is there such an app or am I wishing for a miracle?