I have been to the Thurrock Costco, which is in the same area as the Lakeside shopping mall. I found it to be layed out pretty much the same as those in North America. When I am travelling we like to stop into the different Costco's as well, my wife likes the bargains on clothes.
The Lakeside mall's website has a howto get there section on it's website.
http://www.lakeside.uk.com/