In Costco Southampton branch they had several types of costumes ages 3-4, 5-6. I only looked at the girls ones which were a mermaid, princess, fairy and a pirate lady. They had some boys ones but I can't remember what they were as well as some infant outfits which were cheaper. I'm assuming they're stocked in all branches. The outfits seemed to have a lot of detail/ accessories - great value given most fancy dress outfits are about £15.