I don't think I'll worry about remembering to take that next time I go shopping :)
A bit of an explanation for non-Brits: in the UK, we have several major supermarket chains. The largest, Tesco, only has a market share of about 30%. The other major ones are Asda, Morrisons, Sainsbury's and the Co-operative (who tend towards lots of local convience stores rather than large buildings). The competition between the big ones is vicious and UK shoppers are notoriously fickle. So it's not surprising that a price comparision scheme would generally only net you a few pennies. It has its upsides though - the supermarkets are very good at forcing petrol prices down (simply because your average UK driver will go wherever's cheapest, regardless of branding).
Random trivia: some time ago, Walmart tried to get in on the UK supermarket business by buying Asda. A few years ago they complained to the UK competition watchdogs about Tesco's market share. That's right - Walmart, the US giant, couldn't keep up with the competition here.