Until it's tested in court on a gray area case, it's hard to know for sure, but I'm pretty confident that UK courts would apply the "reasonable doubt" criterion

Until it's tested in court on a gray area case, it's hard to know for sure, but I'm pretty confident that UK courts would apply the "reasonable doubt" criterion to the fact that the defendant indeed posesses an encrypted volume they have not provided the keys to. I.e. if the defendant can reasonably claim that there are no further encrypted volumes on their device, I don't believe they would be imprisoned under this rule.