Under the Defamation Act 1952 (as amended by the Defamation Act 1996), it's an offence to publish a malicious falsehood which is likely to cause financial damage.
Such a 'malicious falsehood' would normally be dealt with in the civil courts (resulting in damages being awarded) but, in some circumstances, it could be classed as criminal libel. The maximum penalty for this offence is two years imprisonment plus an unlimited fine. (If the case was heard before the criminal courts, this wouldn't prevent the company which was libelled from pursuing a case for damages, in the civil courts, on top of the penalties imposed by the criminal courts).
Even if the case is not referred to the criminal courts, libelling a major company can lead to some very long legal battles:
http://en.wikipedia.org/wiki/McLibel_case
Chris