CostCo is mostly for buying items in bulk. You need to be a member in order to enter (either Individual or Trader). As a member you can bring in two people along with you to 'help' with the shopping. We have been to CostCo branch in Watford and in Croydon. You can buy almost anything here, from clothes to office/school materials to food to toiletries to sweets to garden stuff to technology to jewelry. Whatever you want, you name it!
After shopping, I always enjoy eating their hotdog and giant pizza! Oh and refillable softdrinks too. don't forget their ice cream too. Cheap and you're full!