Im not sure about cqout, but im pretty certain you cant combine the uk and the us listings on here as that 577,000 does include uk listings ( i think)
an example, searching for "book" on the us site and limiting to us auctions only generates around 27000 listings, however using all countries and you get over 67000.
Im assuming the us and the uk sites include items availiable to that country, so i would guess that in terms of listings we cant combine the sites as you would be including some auction listings twice.
hope that made sense