The Disney Channel did a movie of this book years ago. This is a cute cozy.
From the back cover:
It was a relief! Stacey wasn't the only person carrying a hatbox on the flight to London. Now she wouldn't stand out like a sore thumb, and no one could possibly guess that instead of a hat she was actually transporting a one-of-a-kind double tiara of diamonds, rubies, sapphires, emeralds, and fire opals. No one could guess, that is, until the jeweled headdress disappeared. By then Stacey was in London--in the company of two impish children and an out-of-work actor--and the tiara was God knows where.
Could it be that the oil baron's wife had switched her hatbox for Stacey's? What about the efficient stewardess, or the beautiful but beguiling cover girl, or the two men handcuffed together? Racing all over London and environs, Stacey was determined to hunt the jewelry down. What she found were lots of empty hatboxes--and murder...
This is a clever mystery that manages to be funny and a bit romantic, too.