A 30-year-old man presents to the Emergency Department with sudden onset severe shortness of breath and pleuritic chest pain. Observations: HR 120 bpm, RR 28/min, SpO2 92% on air, BP 110/70 mmHg. A chest X-ray shows a large right-sided primary spontaneous pneumothorax with a 4cm rim of air. What is the most appropriate initial management according to BTS guidelines?