I shop-on-line with Tesco, and have plenty of time to look for bargains, in their special offers section, and many other items tend to drop in price every other week....
Eg: 8 wraps are usually £1.70p, but drop to £1, Aero mouses mint & choc £1.40p to half price, cereal bars to many to list, chicken breast & sliced bacon packs.
You can also save money by having your delivery's on a Tue, Wed & Thurs, for only £3.00 a month, and can have Tesco Direct items, added to your grocery orders with no extra delivery charges.
I find the Co-op expensive compared, and the service isn't that good.