I am interested in the letter of the law, because that's what matters, not how it's being applied while the winds are blowing in a particular direction.
As to the general question, no, a statement being true does not immunise it from an accusation of it being used to stir up or incite hatred, or at the very least such a defence is not defined within the Public Order Act 1986. We do have the Human Rights Act which protects Freedom of Expression, but whether you could use it or other defences is pure speculation on my part: I would need to see some actual caselaw.
I've attempted my own searches but have only encountered the usual suspects: holocaust deniers and their ilk. Please let me know if you find such a case because I genuinely think that would be interesting to debate, but debating over pure speculation and innuendo is very boring.
To be clear, I haven't even looked, but being a recent topic of debate, it seems important to clearly establish the letter of the law.
> Why is this not just concern trolling?
Because the law-as-written is what matters, like I said, not the law-as-it-has-been-exercised-so-far. Unless you think people inclined to abuse the law will never be elected.
> I've attempted my own searches but have only encountered the usual suspects: holocaust deniers and their ilk
Depending on the specifics, that already seems problematic. There are also chilling effects that are not clearly visible until after the fact. How long have some people wanted to discuss the over representation of some ethnicities in sexual assault clusters, but couldn't because of these laws?
