Costco is a great place. It's a similar set up to Costcos I've been to in the States, but obviously there is a UK slant on the products for sale.
Kirkland brand doesn't feature as heavily here as in the US, but there's still lots of it around. You really must go with a list, and an idea of the comparable prices at a 'normal' store for the items you want, or you could easily be paying over the odds for the privilege of buying in bulk.
For me personally, we have a set of things that we tend to buy that are either good value, or better quality than at the supermarket, so worth the extra cash. I portion things up with Ziploc bags (bought at Costco, natch) and freeze it, and it works very well.
1) Loo roll - the Kirkland one. When it's on offer, it's a great deal. They also do triple Velvet, which when it's on offer, is a good price
2) Chicken breasts - more expensive than the supermarket, but they're bigger, and there's generally no nasty bits to chop away. They run around £1.50 a breast, but I find one breast feeds 2 of us.
3) Angus mince - definitely more expensive than the supermarket, but so so so much nicer. No grizzle, low in fat, great texture. Their normal mince is cheaper and is fine, but not as nice as the Angus (look for the black foam container)
4) Kirkland hot dogs - in the big chiller section (ie, not by the deli stuff). Same as what they sell in the café. I wrap them up individually in clingfilm and then put them all in a big Ziploc in the freezer.
5) Frozen chicken strips - delicious, great for a quick dinner or snack, last forever
6) Herbs & spices - huge containers that you won't get through before they go out of date, but even if you only use a quarter of it, it's cheaper than the supermarket. Best of you can share with a family member or 10.
7) Kirkland vanilla - great quality, great value, worth getting
8) Kirkland chocolate chips - if you're used to American choc chips, you'll be happy to find this. Huge bag, makes baking cheaper!
9) Two litre bottles of Pepsi - around 80p a bottle. Normally can't get it for cheaper than £1 a bottle even in a sale at the supermarket
My father in law likes the danishes, the part baked bread rolls, and cod liver oil. I got some name-brand sunglasses that were £110 at Boots for £70 at Costco. I could easily go here and spend a couple of hundred pounds. 48 packs of Kirkland AA batteries for around £10. A normal trip with the list of things I mentioned above runs us around £100.
The downsides? 1) Queuing. You'll be standing around a lot. 2) You have to be eligible to be a member, or know someone who is that will take you along, and it costs £30 a year. 3) Everything is huge. We wanted some pickles, but the only jar wouldn't have fit in my fridge (but was only £3.50). 4) Location - we don't have a car, and it's a trek to get to. We tend to wait until someone else can give us a lift, or we take a minicab home for around £15, which negates some of the savings but still works out for us (you can Uber from Costco Watford). 5) Cash, debit card, cheque, or Amex only. If you're a guest of someone with a card, they technically have to pay for you. In reality, you can pay cash. read more