Comment by toast0

I don't know about Costco UK, but there are many staple items where the 4-pack of whatever costs about 1.5 times the single pack at my local mainstream grocery store. There's also a lot of items where the local mainstream grocery store tends to have something in that category on a loss leader sale below the costco price, so if you're brand insensitive or have room to stock up when your brand is on sale, you'll do better at the mainstream grocery; since you're also shopping at costco, probably at least one of those is true. And yeah, if you have particular tastes in certain categories, chances are you need to go somewhere else.