The disagreement about a comparison to Nazis isn't about what the Nazis did, or that doing what they did would justify comparison, it's that I fundamentally disagree that taking down an allegedly libelous document is the same as mass murder.

I'm familiar with the horrors of the Nazis, you don't need to rehash.

I'm not sure how a German court case about denying Nazi crimes has bearing on a theoretical UK court case about if an organization has a specific policy position.

You earlier mentioned that the case could be a PR and fundraising opportunity.
Have you considered if you would want them to fight it if they thought it would do the opposite? What if they just spent a bunch of money giving the assholes a platform to spew hate?