I've only heard about UK libel laws -- as much from the Leon Uris book as anything else. But there also seems to be a tone that anonymous posters should be permitted to say anything they want. As I understand it, libel only applies to statements of fact, and not opinion, and truth is still a defense. If so, anonymous, or identified, posters are free to state opinions -- not facts -- and if facts are posted, then stick to the truth. If the posting is just a rant, I don't see that it accomplishes much.
For whistleblowers, I don't know enough about UK law. Certainly there have been abuses. Scientologists, for example, filed a bunch of lawsuits when their procedure manual became public. I don't remember how it ended on the copyright issue, but they couldn't claw back the truth.
And what about nuts? It may be bad PR to file a lot of suits (look at the record industry) but there are certainly some boneheaded rumors out there. Procter & Gamble is satanist because of their logo? Hogwash, promulgated by nuts, but persistent. Urban legends abound and if they cause harm to a reputation, I don't see a huge problem in gathering facts using proper means. As to what they do with the facts, it sounds like everybody's guessing. There may be dozens of reasons to decide to not sue, but until you have the background facts, you don't get to that decision.