Yes, but the point I'm getting at is whether someone could hypothetically still be outside the law if they gave access via the first password, but the interrogators suspected (but could not prove) that a second volume existed.

They don't need to prove anything. They just imprison until you reveal the password for the hidden volume. And if there's really no hidden volume, then you're SOL.

Max tariff in the UK is 2 years, or 5 in Terrorism cases. Does anybody have any actual insight on this specific UK law, rather than guesses?

Once again, this is in context of UK law. So speculation about rubber hoses in dungeons isn't particularly relevant (luckily).