I don't know the answer. I suspect it wouldn't be a defence in the UK. However, a slight variation of this is a tactic Trump uses a lot. He'll post a statement on the lines of: "Many people are saying that....[insert outrageous statement here]." Would that work here as a way of posting defamatory statements? Again, possibly not if the person being mentioned has lots of money. One might be asked to show to the court who these people are.