You are wise to consider the posibility of defamation claims. I am a California attorney, so I can't help you with UK legal requirements. In the US, reviews are generally not considered defamation unless there is a clear misstatement of fact. How exactly this is determined depends on the state that has jurisdiction over the lawsuit. Regardless, many courts are holding the site, ISP, etc., liable as "secondary publishers."

I did a quick search for potential UK sources. I can't tell you if these references are credible, but they might help.