I wasn't talking about the party. I was referring to potential criminal charges brought by the ICO or civil defamation action brought by people named in the document.

And "public interest" could be a legitimate defence for publishing some snippets or a redacted version of the document, i.e. as some media like Sky News have done. It would not at all be a serious defence for publishing the entire document with the private information of a significant number of potentially vulnerable people and/or minors, as is the case in the document. The ICO have multiple times now in court completely taken down people who have tried to claim such spurious use of the "public interest" defense - there is a high bar to clear there and it's not a get out of jail free card.

When it comes to defamation, of which I imagine there will be quite a few cases resulting from this publication, the libel laws are pretty clear on the fact that the onus is on the defendant to prove their claims, and not on the complainant to prove them negative. Which leaves people publishing this document in a very precarious situation, as it will be in many cases extremely unlikely they will have any significant evidence available to them in order to prove the claims made in the document.