The other option is that it is the equivalent of a numbers station. MI6 write the columns, and the only parts that are important are a few letters at prearranged positions, which form the next cryptographic key to be securely distributed to agents in the field. Since the adversary expects British spooks to be more Times/Telegraph types, it’s more deniable.
I'm convinced that Chiles and Tim Dowling have some kind of blackmail on the owners of The Guardian. It just makes no sense otherwise.
save