She really is bonkers, isn't she. Is she really likely to win any action - there is the defence of honest opinion, which I think could be robustly relied upon. My understanding is that a defence on those groups needs to be based on the following:
The statement was an opinion
The statement indicated the basis of the opinion
An honest person could have held the opinion based on any fact that existed at the time
Starmer expressing the opinion that Truss crashed the economy would seem to fit very nicely into those criteria. But I wouldn't want to challenge this if I were Truss as it would shine a spotlight on the 'facts' that existed at the time - in other words the chaos that followed the mini-budget and the impact on the economy that ensued. If I were her I'd rather prefer not to rake over the embers of the shambles.
If the court accepted it, which I doubt, she'd lose. Hiw does she show that she didn't crash the economy? As jeremyp has highlighted, it's not really clear what that would mean so I can't see how you show it to be false.
And as you highlight it's clearly opinion in terms of what Starmer is saying, and I think all that you would need to do is say that the opinion is validated by her party effectively getting rid of her.
As noted earlier, I think it's an odd thing for a supposed supporter of a liberal view of free speech to go after. If it were to succeed,I suggest that Truss will have made numerous statements that would then be subject to successful claims, as would pretty well every politician.
It feels that for her the only thing worse than being talked about, is not being talked about enough