this is the traditional loophole.
The law does not prevent you from using a device to detect the presence of signals / transmissions.
The law prevents you from using wireless telegraphy apparatus with INTENT to obtain information as to the contents, sender or addressee.
so if you are using the apparatus for the detection of signals say for DF work and you accidentally overhear stuff you are not breaking the law.
Then of course Offence 2 steps in to prevent anything you accidentally overhear from being repeated.
so the defence goes something like, " i was using my scanner to detect if their was the presence of radio transmissions in this vicinity, and it stopped on a frequency and i accidentally overheard stuff which i have kept to myself honest your Honour".
GB