https://www.bailii.org/ew/cases/EWHC/QB/2021/85.html
If I had to summarise, I would say that this is a case of a consumer doing the most emotionally satisfying thing in response to receiving a poor service, rather than what is legally allowed. As the judgement says, the defendant never attempted to complain or ask for a remedy from the solicitors themselves as he is legally entitled to.
> "Also it was apparent that the Defendant made no attempt to engage in the Claimant's dispute resolution process. A complaint should always be the first stage in resolving any issues of customer satisfaction."
The judgement also explains that in order for the defence of "speaking the truth" to succeed, he would need to submit evidence that others had been scammed by the same company - apparently the evidence of just a single datapoint is not enough:
> "I recognise that in the context of an application for summary judgment I must not conduct a mini-trial but it seems to me the Defendant's allegations are so bold that in the complete absence of any credible material to support them I can conclude there is no real prospect of them succeeding at trial. I also take into account that the highpoint of the material submitted by Defendant is that he is unhappy with the service provided by Claimant."
So while it seems that in certain cases, the correct response to receiving a poor service would be to leave a negative review, it seems the law does not agree.