It is possible that the paper sold by Costco in the UK is different than the one sold in the US. The last box if the Costco Kirkland glossy paper I bought in the US, says that it was "Made in U.S.A.". It is also 255g/m2. Where is the paper they sell in the UK made?
I can confirm, however, that the US Costco glossy paper is indeed an amazing bargain.