That only works if you can threaten to put the owner into jail for not complying. If you're trying to spy on communication between two people outside your jurisdiction, you're out of luck.
(that doesn't mean I support the US governments attempt to undermine secure communication)
Somehow, an attacker can have your whole conversation log (encrypted), including the key exchange, and be unable to retrieve the key used, even if he has the credentials you used at the time the key was generated.
The real crux of the question may be, "How does Diffie-Hellman work?" (Well known key exchange method.)
Theoretically, let's say you use VeraCrypt or something similar to create a password-protected volume inside a big .mp4 file using steganographic techniques. A second secret password is set, which can decrypt another hidden volume which contains the actual sensitive data.
You give out the password for the first volume under duress.
Would that be enough to satisfy the law? If so, isn't it undermined?
Once again, this is in context of UK law. So speculation about rubber hoses in dungeons isn't particularly relevant (luckily).
I don't think somebody living outside of UK cares. The threat only wors if that person lives in UK jurisdiction. Not everybody does.