Is it illegal in the UK for two or more people to communicate in such a way as for the communication to be not understood or recorded by an external party? If

Is it illegal in the UK for two or more people to communicate in such a way as for the communication to be not understood or recorded by an external party? If not, how is it illegal to facilitate that communication? (I guess: because authoritarian politicians can make up nonsensical authoritarian legislation) I prefer legislation to be sensical, but I guess this is the timeline.