Double jeopardy was partially eliminated in the UK in 2003 for qualifying offenses. I don’t think this has been tested, but during a retrial, a refusal to provide a password would be a separate RIPA offense from any refusal during the first trial. So you could actually be jailed more than once for this. For qualifying offenses all that is required for a retrial is “new and compelling evidence” which is a low hurdle for politically unpopular defendants.

If that did happen you'd have a good case from the Human Rights Act because it becomes indefinite imprisonment. The UK is still following the ECHR as well.

But arguing these theoretical untested-because-they-never-happen edge cases isn't exactly pushing forward a good case for this law having been "badly written." There's seemingly no problem with it in practice.