As others have indicated, there is no requirement for a police officer, with a handheld speed gun, to stop a motorist in order to issue a 'ticket'.
The police often used to base themselves, with speed guns, outside my late father's house. On a sunny day he'd sometimes wander out to chat to them. They told him that they would occasionally stop a driver if they only wanted to warn him (rather than issue a 'ticket') or if they thought that his speed presented an immediate risk to other road users. (In particular, they would stop speeding drivers around times when children were going to or from school).
However, they generally preferred not to stop speeding motorists because, while speaking to motorist A, speeding motorists B, C, D and E might go by without being caught. There could also be times when, for their own safety, they deemed it unwise to step out in front of a speeding car.
Chris