Ms Cadwalladr is running a public interest defence and is not alleging that her statement was true.
I had thought that this really didn’t exist in English law. But it does:
Publication on matter of public interest
(1)It is a defence to an action for defamation for the defendant to show that—
(a)the statement complained of was, or formed part of, a statement on a matter of public interest; and
(b)the defendant reasonably believed that publishing the statement complained of was in the public interest.
(2)Subject to subsections (3) and (4), in determining whether the defendant has shown the matters mentioned in subsection (1), the court must have regard to all the circumstances of the case.
(3)If the statement complained of was, or formed part of, an accurate and impartial account of a dispute to which the claimant was a party, the court must in determining whether it was reasonable for the defendant to believe that publishing the statement was in the public interest disregard any omission of the defendant to take steps to verify the truth of the imputation conveyed by it.
(4)In determining whether it was reasonable for the defendant to believe that publishing the statement complained of was in the public interest, the court must make such allowance for editorial judgement as it considers appropriate.
(5)For the avoidance of doubt, the defence under this section may be relied upon irrespective of whether the statement complained of is a statement of fact or a statement of opinion.
(6)The common law defence known as the Reynolds defence is abolished.
That’s seems to give coach and horses room for journos to libel but then IANAL and it might all be more subtle than I thought.