As in the U.S. we do have quite a wide selection of sizes in the U.K, and 1shts are among them. The Quad is the most popular, possibly followed by the 1sht.
Also sometimes the sizes are not bang on, and in some cases inches out. This is not all the time, but it is not unusual. So unless the poster is in front of you, in some cases it is difficult to say if it's genuine or reprint. It does catch a few out.