Unauthorised eavesdropping on communications using wireless telegraphy apparatus and/or the disclosure of information acquired in this way is a criminal offence - The Wireless Telegraphy Act 2006.
Regulation of Investigatory Powers Act 2000 makes it a criminal offence to intentionally and unlawfully intercept communications by post, phone or other telecommunications systems. This would include intercepting voicemails on a mobile phone without lawful authority (phone hacking).
These laws do not affect journalists recording telephone conversations to which they are a party either for research purposes or with a view to broadcast. But see regulatory provisions regarding privacy considerations 'Privacy'.
Accessing a computer without proper authorisation (computer hacking) is also a criminal offence – the Computer Misuse Act 1990.