Truth is not a defence.
Actually it can be a defence in libel. It's not necessarily a defence in the tort of misuse of private information which has just about replaced libel as a cause of action. This is mainly because a libel action can only be launched in the high court and costs a fortune. So if you ever hear about someone winning a libel action, it's a 99% certainty it's actually a MoPI case.
It's interesting as Thornberry frequently threatens libel actions and doesn't follow through (she's threatened Private Eye for example). But she has also sued, and won, against a few Islington Lib Dem councillors in the past.