Nobody has yet found a way to achieve real-time decoding of the encrypted signals of the Airwave system which is now in use by all UK police forces. As the encryption is considerably stronger than that used by satellite TV broadcasters (which is fairly difficult to 'hack'), it's unlikely that the security of the system will ever be breached.
It's an offence under the Wireless Telegraphy Act, 1949 (as amended by countless other pieces of legislation) to listen to any radio transmissions, other than those from authorised broadcasting stations and licensed radio amateurs, unless the listener has authority to do so.
(Back in the days when the emergency services used unencrypted transmissions at the top end of the FM broadcasting band, a group of 'fire chasers' used to listen to the fire brigade broadcasts and travel to the scenes of fires. They were arrested and the court subsequently confiscated both their radios and the cars which they were fitted in).
Additionally, anyone who records (whether electronically or in writing) the content of any police transmission, or conveys the content of that transmission to any other person, will be acting in contravention of the Official Secrets Act. (In certain circumstances, there may also be a breach of the Terrorism Act, 2000).
Chris