Tardigrade doesn't have the key, either. Only the owner has the key, and that's neither you nor Tardigrade.
The core of your assertion fails because it's not actually the "you versus plod", but rather your brief versus the DPP's, and as a matter of fact the plod would have to show that they have:
"reasonable grounds—" .. "that a key to the protected information is in the possession of any person,"
(RIPA, Section 49, Section 2, subsection (a)).
Your suggestion is that the "reasonable grounds" requirement is trivial to show. It isn't -- Apple is the poster child for this, as they have backups from millions of phones and tablets to which they have no access, due to not having the key, and the plod having no reasonable ground for believe they do.