According to Is it illegal for Joe Public to listen to ATC in the UK? over on Aviation.SE, although aviation transmissions are broadcast publicly, in the clear, unencrypted; it is illegal to listen to them live on a scanner or such.
What law prohibits this? I believe it dates to WWII fears of spies. I also believe the UK is the only country with such a law. You can freely listen to New York's JFK or Amsterdam Schiphol's ATC without a problem.
Has anybody been successfully prosecuted under this law?