If you use a scanner to obtain information contained in a message of which you are not the intended recipient, it's an offence under the WTA.
And the CPS would take one look and prosecute? Ever? How many Policeman pass the stands selling air band scanners at air shows? I have lost count of the occasions I have chatted with Police with a scanner earpiece in. They don't care and rightly so.
But to be clear, all technically illegal. With no prosecutions ever or likely in any reasonable scenario. Much of the criminal law can pass into disuse or only be used in quite exceptional circumstances.