Obscure YA sci-fi book from the mid-1980s

OK, I often think I'm the only person in the world who ever read this, but as it was a hardcover at my rural KY library, it must have had fairly wide distribution.

I cannot (of course) remember either title or author, but I do know the author was British from her spelling conventions. The book was about a preteen girl named Valerie, called "Valley" for short. She was from a relatively advanced society with FTL travel and her family were explorers, but she and her brother were abducted by aliens whom she called "the popeyes" because of their eyestalks and used as a cultural study/experiment. I believe she was trying to figure out their teleportation tech to get back to her family.

It was an excellent book. Any help would be useful.