A 35-year-old teacher presents with a 6-week history of low mood, anhedonia, poor sleep, and reduced concentration. He scores 18 on the PHQ-9. He has no past psychiatric history and no suicidal ideation. After discussing psychoeducation and low-intensity psychosocial interventions, he wishes to try pharmacological treatment. According to NICE guidelines, what is the most appropriate first-line antidepressant?