Seems to me like coordinating with an entity outside of the spooks' control, such as the BBC, would give more opportunities for leaks. It would also reveal some information about who is controlling the signal--someone with some kind of relationship with the broadcaster.

During WWII, the BBC would daily have a section after the news dedicated to "personal messages" - which everyone knew were instructions to the resistance in France, or similar. "William waits for Mary" was one of the more famous ones related to D-Day, I think.