"Someone can absolutely be prosecuted under the Wireless Telegraphy Act for receiving something they shouldn't that was unencrypted and it has happened. An offence under the Regulation of Investigatory Powers Act has probably also been committed if someone knowingly intercepts private messages intended for someone else without their permission. "
Anyone remember the days when the Police radios used VHF/FM and were broadcasting just above the frequencies allocated to public broadcasting? Quite a few commercial radio receivers would tune just far enough out of band that you could hear the Police. Not that I ever did, of course, no sir, not me.