I have heard a number of people say this but I've always wondered which bit of legislation is involved. Just curiosity really, but can you explain?
UK Wireless Telegraphy Act 2006:-
48 Interception and disclosure of messages
(1) A person commits an offence if, [without lawful authority] —
(a) he uses wireless telegraphy apparatus with intent to obtain information as to the contents, sender or addressee of a message (whether sent by means of wireless telegraphy or not) of which neither he nor a person on whose behalf he is acting is an intended recipient, or
(b )he discloses information as to the contents, sender or addressee of such a message.
(2) A person commits an offence under this section consisting in the disclosure of information only if the information disclosed by him is information that would not have come to his knowledge but for the use of wireless telegraphy apparatus by him or by another person.