A 30-year-old man presents to the Emergency Department with sudden onset shortness of breath and pleuritic chest pain. He has no significant past medical history but returned from a long-haul flight yesterday. His observations are: RR 24/min, HR 110 bpm, BP 120/80 mmHg, SpO2 93% on air. His Wells score is calculated as 5 (moderate risk). According to NICE NG158, what is the most appropriate next diagnostic step?