The Transporter playset. This was an item that Playmates complained about that it had overproduced, or been forced to overproduce, and that was later under-ordered by stores, so that it was stuck with tens of thousands of unsold items in their warehouse, to the point that it lost money on it. The culprit, of course, was the shelf space required -- stores didn't want to offer tons of shelf space to something that was not a ship, and required some dough.
This is part of the reason why we never saw anything after the Bridge and Engineering playsets -- you'll note that Engineering was produced in much smaller numbers than the Transporter and the Bridge. This is also part of the reason why we never saw a TOS bridge, as it would have been too expensive to do properly, versus in a shoddy but passable manner.
It's tough to tell how popular The Transporter was in the UK, or in Germany or France for that matter, but let's just say that the UK got the bulk of these from what I've heard.