Opinion has to be backed up by facts and you have to be able to prove those facts. Defamation Act 1952, section 6. http://www.legislation.gov.uk/ukpga/Geo ... tification
The meaning is explained in a Supreme Court Ruling of 2010.
"Third, the comment must be based on facts which are true or protected by privilege: see, for instance, London Artists Ltd v Littler  2 QB 375, 395. If the facts on which the comment purports to be founded are not proved to be true or published on a privilege occasion, the defence of fair comment is not available."
"It is, however, plain that certain statements which might on their face appear to be expressions of opinion (as where, for example, a person is described as untrustworthy, unprincipled, lascivious or cruel) contain within themselves defamatory suggestions of a factual nature. Thus the law has developed the rule already mentioned that comment may only be defended as fair if it is comment on facts (meaning true facts) stated or sufficiently indicated."http://www.supremecourt.gov.uk/decided- ... dgment.pdf