A couple of people have been convicted of refusing to hand over their encryption key.
It's worth noting that this is a separate offence, so there's a determinate prison sentence. You can't be held in contempt of court for refusing to hand it over.
You'd have to keep it on a separate storage medium. And if you have to hand out the done medium, what's preventing them to get your second medium? And if you are able to keep that second medium secret and safe, why don't you store the whole unencrypted data on it in the first place?
Either way: OTPs are really cool, but I don't think they have any relevance here.
Cheers.
"Of the 15 individuals served, 11 did not comply with the notices. Of the 11, seven were charged and two convicted."
Whilst they may have been incarcerated since the report, at the least it would seem that there is some evidence based procedure to determine "guilt" in terms of whether you are able to produce the key or not.