I'm pretty sure that you don't even have that defense in the UK. I think the standard for defamation there is that you made the statement with the intention of harming the reputation of the person you made it about - even if you believe the statement to be true (or even if the scientific/historical consensus believes the statement to be true), and if you believe that their reputation should be damaged because of the thing that you believe to be true.
So the best (UK) defense seems to be to claim that you incidentally harmed someone's reputation with information that you believed to be true, while attempting to do something other than harming that person's reputation.
This makes it sound like public criticism is virtually impossible in the UK, as publishing negative information invariably impugns their reputation in somebody’s view. Or at least that case can be made trivially in court.
This makes it sound like public criticism is virtually impossible in the UK
No, it just means your right to public criticism is dependent entirely on the public opinion of what is and isn't off-limits. You can say pretty much whatever you want as long as you don't direct it at the wrong person. This is of course a situation most Americans would find horrific.
Well, no, that's not really how it works. In this case of course the "wrong person" would be someone who can prove that what you are saying is incorrect.
That's approximately correct. Public criticism is very, very difficult to do safely in the UK and that's for the lighter fare. The same is also true in, for example, France. Public criticism of officials (eg police) is legitimately dangerous and can easily get you a jail sentence.
Firstly the plaintiff must satisfy the judge that the specific act of defamation could have caused actual harm to their reputation. (This is probably the cause of your confusion, but it's a new bit actually intended to make it harder to sue for defamation/libel than before). If they don't do that, there's no case to answer even if the claims were tendentious and malicious.
If the plaintiff succeeds in that claim, the defendant can attempt to use the defence of honest opinion (or alternatively prove that on the balance of probability that the claim was true)
Obviously that's a tougher law on defendants than jurisdictions where the plaintiff has to prove that the claim is false and prove that the defendant knew it was false, but it's still quite easy to intentionally harm people's reputation by publishing things which are true, or simply a matter of opinion about their general character.
So the best (UK) defense seems to be to claim that you incidentally harmed someone's reputation with information that you believed to be true, while attempting to do something other than harming that person's reputation.
So NZ seems fairly progressive in comparison.