I suspect MI6, or any other major intelligence agency, would be smarter than that, and have much better tools at their disposal.
There's nothing magical about these radios. Yes, they'll do encryption, all the way up to FIP-140 AES-256, but just about any LMR radio is capable of that either via software or internal encryption module. Heck, even LTE cell phones have encryption. You don't need a fancy radio for that.
This is either legit hams that made a poor choice, or someone try to smuggle something. Like most things, I'm sure there is a lot more to this story.