Well all I could find was this:
"In most cases, this practice is legal as, unfortunately, British citizens are not protected by a privacy law. An act of 1985 makes the 'international interception' of communications illegal, but this is primarily aimed at unauthorised telephone tapping.
Significantly, the Post Office Act 1953 prohibits the opening of 'any postal packet' which ought to have been delivered to another person. In practice, however, this protection for private mail is limited since an offence only arises if it is opened 'wilfully and maliciously with intent to injure another person'. Needless to say, this can be very difficult to prove."