A 60-year-old woman with known severe COPD (FEV1 35% predicted) who is on long-term oxygen therapy (LTOT) calls her GP complaining of increased breathlessness and sputum purulence over the past 2 days. She is normally managed at home. Her current oxygen saturation is 89% on her usual 2L/min of oxygen via nasal cannulae. She has a self-management plan including 'rescue' antibiotics and steroids. According to NICE guidelines, what is the most appropriate immediate action?