The Army doesn't want them, they're too much trouble A missing agent, kidnapped and taken to hostile territory for interrogation. A 'do or die mission' where the stakes are high and failure isn't an option.
MI6 does, they're the best at what they do, and they need the best.
MI6 have a problem.
Only one option is open, and that's a high-risk rescue operation over two hundred miles into hostile territory with no support.