The investigator says "he will die in due course", which of course we all will, but that wording makes me think it is someone old, where the investigator expects to live to see the truth exposed. And you're right, the words national treasure don't just mean someone popular, it's used to mean an old person with a long career, who is seen as loveable and can do no wrong.
If Prince Andrew is not untouchable, but RH and JS were, then it makes me think that it's owing to media blackouts and fear of speaking, rather than anything legal. I wouldn't be surprised if the person in question is untouchable because they have a huge amount of dirt on lots of people. I've seen a clip where Prince Charles and Princess Diana are meeting Jimmy Savile, and Charles says "How on Earth do you manage to raise twelve million pounds?" BY HAVING DIRT ON A LOT OF VERY RICH PEOPLE, CHARLES! Savile's access to finance goes way beyond the scope of people sponsoring him to do marathons, someone very big was bankrolling him. He used to have local high-end cops over to his house for private hangouts every weekend. Presumably they didn't go there every week for the witty banter. Very probably Savile would procure young girls/boys for them, and so he had big dirt on them.