My understanding is that defendants in Britain have to prove
statements true by the preponderance of evidence, whereas in the U.S.
the standard of evidence is "compelling" (a lower standard).
This is not the case. Preponderance of the evidence can still be the burden of proof in the United States (in a civil libel case, although it must be proof beyond a reasonable doubt in a criminal libel case).
But, the difference is in what has to be proved. In Britain, it appears to be necessary to show that the statements are true in order to prevail.
In the U.S., it is merely necessary to show in a case like this one (because it involves a matter of public concern) that the statements were made with knowledge that they were false, or with reckless disregard to the truth or falsity of the statements made. A factual basis for believing the statements made to be true is a defense if the basis is at all reasonable, and is a complete defense if the factual basis for making the statement is disclosed and that is true or believed to be true by the speaker.
It is not necessary for the statements to actually be true under U.S. law in a case such as this one, although actual truth is also a defense, which is not the case in all circumstances in U.K. law, and was not the case under the historical common law. Historically, defamation claims could be brought for statements critical of the monarch, for statements pointing out the natural infirmities of someone for example by mocking a person with low IQ, or for speaking ill of the dead.
Furthermore, the U.S. has a variety of doctrines that make it hard to find that a false statement was made in the first place.
For example, statements of opinion are not actionable and many of the alleged falsehoods in the McLibel case would be considered to be statements of opinion in U.S. law rather than statements of fact.
Similarly, U.S. law does not require that statements be literally true, and instead recognizes that a defendant may have been engaged in using hyperbole, or may have gotten the gist of the accusation right even though strictly speaking the exact statement made is not technically true (e.g. someone might say that a company paid a "penalty" when it actually paid a settlement amount in a lawsuit seeking a penalty or paid an amount representing compensatory damages only rather than a penalty amount).
In the same vein, it must be clear from the context of the statement that the person making it intended it to be received as a truthful account and not a mere parody or satire which was intended to be understood as false.
For example, I couldn't sue someone who made a knowingly false statement that I assassinated King George V, who died several decades before I was born, or that I was telepathically controlling my uncle because I had a space alien parasite in my spine. Those claims are so absurd that they would be inferred to intended to be fictional on their face.
Certain kinds of falsehoods (e.g. lying about one's military record in a a political campaign) are simply not actionable as a matter of law, no matter what, as the harm is not concrete enough.
There is not, however, necessarily a defense under U.S. law to defamation liability if the defendant said many things that were true, but something else that would be defamatory in isolation.
For example, even if everything else were true, if the defendant had also stated that the CEO of the Plaintiff was convicted of leading a Nazi concentration camp and killed millions of people, which would have been possible given the CEO's age, knowing perfectly well that the person with a similar name to the CEO who did so was someone else who died an untimely death decades ago, that statement might be defamatory and actionable (at least by the CEO personally and probably by the company if it was alleged that he was hired despite the fact that the company was aware of this circumstance).