Googling around, I found that Jane Preston was a pseudonym of Reginald G. Thomas. He also wrote as Judy Thomas. During the 1930s he wrote boys' adventures under his own name, mostly for The Ranger and Chums. In the early 50s, using the female names, he wrote long text stories for Girls' Crystal and School Friend. I presume these stories were adapted for SPL, but I haven't found confirmation.
Biographical note:
Thomas, Reginald G(eorge) (1899-1956)
Born in Worcester; educated at University College, Wales; a journalist.
Source: philsp.com
Link to the book:
Schoolgirls' Picture Library 073 - Their Amazing Chinese Journey