Good faith (that is, having evidence from which one would reasonably believe the truth of the statement at the time it was made) is not a defense to libel in British law.
Libel in the UK is a strict liability tort, the statement can be 100% justified based on the information in hand when it is made, and even if the court would agree with that were it permitted to consider it, if the court finds it is false based on information that the defendant did not have (or even could not possibly have had, no matter how hard they sought it) at the time of the statement, the defendant will still be liable.
EDIT: while the NY Times v. Sullivan rules for public officials (extended by other cases to public figures) get more attention, the Gertz v. Welch rule that, as a matter of Constitutional law, requires fault for defamation liability for private figures is arguably more fundamental difference between the US and UK on defamation law.
Libel in the UK is a strict liability tort, the statement can be 100% justified based on the information in hand when it is made, and even if the court would agree with that were it permitted to consider it, if the court finds it is false based on information that the defendant did not have (or even could not possibly have had, no matter how hard they sought it) at the time of the statement, the defendant will still be liable.
EDIT: while the NY Times v. Sullivan rules for public officials (extended by other cases to public figures) get more attention, the Gertz v. Welch rule that, as a matter of Constitutional law, requires fault for defamation liability for private figures is arguably more fundamental difference between the US and UK on defamation law.