United Kindgom
During the COVID pandemic, police officers have taken to fining members of the public who break government guidelines (exercising too far away from their houses, exercising more than once a day, etc). It has been reported numerous times (and is clear from reading the text of the law), that none of these behaviours are actually illegal, they are simply government guidelines.
How can police officers enforce something that is not stated in law, and, therefore, and these fines lawful?