I too am a Costco fan and made the trek to the Costco in Reading last time I was in the UK (November).
It was very similar to the US stores, but with differences that made me really sad to live in the States - especially the dozen-packs of Pain au Chocolat and the HUGE PACKAGES OF PROPER CHOCOLATE (I think American chocolate is crap, sorry Hershey's fans).
They had lots of things that were exactly the same, but in all I wished I had gone by car and not by train :-(
Incidently, if you are without automobile and want to go to the Costco Reading, take a non-stop train from Paddington to Reading, exit the station and look at the bus schedule right out front. The buses queue up right there and you can get to the Costco in under 10 minutes