In Britain, when a doctor refers to being "in surgery" that also refers to their regular office hours, not just the American meaning of performing surgery. As a GP, Pru would refer to her office hours that way.
The specific occupation of (Dr.) Pru Carew changes frequently. Slade calls on her to work as a psychologist to help the kids in the shelter. Later she is shown working as a general practitioner examining Mrs. Harrison for a breathing issue. Then in episode four she mentions that she has to leave to perform surgery later in the day. These are three entirely different jobs that no one physician would hold concurrently.