Defamation
An individual can sue for damage to their reputation caused by material broadcast or published to a third party, this includes online.
This area of the law is called defamation or libel. It is the biggest legal pitfall relating to the BBC's output, with serious financial consequences if we get it wrong.
Any living individual or company can sue as long as they are reasonably identifiable from what is said and the material is defamatory of them. This means that it would tend to lower them in the estimation of right-thinking people generally.
The claimant does not need to show that they suffered any actual damage, nor that what was said was false. On the contrary, the defendent generally has to prove that it was true.
If involved in an action for defamation there are a number of possible defences, but generally you will need to have good evidence to back up what you say.