A 45-year-old teacher presents with a 3-month history of low mood, anhedonia, poor sleep, reduced appetite with 3kg weight loss, and difficulty concentrating at work. She feels worthless and has had fleeting thoughts of self-harm, though no current plans. She has no past psychiatric history. What is the most appropriate first-line pharmacological treatment according to NICE guidelines, if psychological therapies are declined or not available promptly?