In talking with the Brits, some I talked to wondered why the military command didn't use the Travelers. These are the tinkers/scrap-metal men, and traveling itinerant day labor folks who are sort of like Celtic gypsies in the UK. Their language is loosely based on Gaelic, but with lots of substitution and transposition mixed in - deliberately to confuse any eavesdroppers already. It is nearly impossible to learn unless you live with these people for a long time. It's not written down anywhere, they're a highly insular group, and they're very attuned to when someone who isn't one of them tries to speak their private language.