From your own links, the monarch has a rights to encourage and warn. How would a monarch go about encouraging or warning about a course of action without forming, and then expressing, an opinion on that matter?
My understanding is the monarch must not become actively involved in government... thus declining to vote, staying out of European Parliament, etc. That's not quite the same thing as never expressing an opinion.
My understanding is that most people would suggest that that only applies to the head of state (currently queen Elizabeth)
But Prince Charles isn't the head of state... So assuming that you're suggesting that this principle doesn't just apply to the head of state, where do you cut the line? Should everyone in the royal family be barred from expressing their opinion privately or publicly?
My understanding is the monarch must not become actively involved in government... thus declining to vote, staying out of European Parliament, etc. That's not quite the same thing as never expressing an opinion.