I shop online and have used both Asda and Tesco for this.
For the past 6 months Tesco have always got my order because by using this price comparison site my order always has worked out to be cheaper at Tesco
mySupermarket - Compare Supermarket Prices – MySupermarket the supermarket comparison site
AND the Tesco delivery service is better - rarely get told 'items not in stock' and better quality too!!!!