Further to PP's posts, the law has been clarified since the Defamation Act 2013 took effect in 2014. It now clearly states "A statement is not defamatory unless its publication has caused or is likely to cause serious harm to the reputation of the claimant".
So you can like as much as you like about someone, even shouting your lies from rooftops and getting them broadcast on TV and radio, as long as you don't cause serious harm to their reputation. Simply annoying them, upsetting them, embarrassing them or making them look foolish isn't enough to risk being successfully sued for defamation.