Also toiletries, laundry and cleaning products.
If you're already a Costco member, I think their toilet rolls are probably the best in terms of price/quality as the rolls are longer than average. The price per roll is comparable or possibly even slightly higher (£16 for 40) than the Amazon price quoted but the Amazon ones might have fewer sheets, so check carefully.
As for the other things, you either need to have a good memory, or do some research into unit prices for the things you buy. Trolley.co.uk is good for comparing supermarket prices.
If you buy basmati rice, the 5 or 10 kilo bags often work out much cheaper per kilo than smaller packs - last time we were in Costco they had 10 kilos of Tilda basmati for £14, I didn't buy it because I wasn't sure it was a good price, but checking later, it was.
Costco is also good for things like eggs although limited stockpiling opportunity, some canned goods but by no means all, herbs and spices if you go through them quickly.
But you don't have to spend all the money at once, you could use it to drip feed into a more cost effective approach to grocery shopping.
We shop for the storecupboard and freezer, not what is to be used week to week (except fruit, veg, milk, eggs) and buy multipacks of non perishables when on offer, so the price per unit is often half what it would be if you bought standard sized packs as and when they were needed them. We also get larger packs of meat and split into meal sized packs for the freezer.
Once you get into the swing of things, you just start looking out for another offer when stocks start to run low, so it can be topped up before you're forced to pay full price. You can also spread the cost, so tend to buy laundry detergent one week, toilet paper the next, rather than buying a small quantity of everything more often.