A 25-year-old woman with known asthma presents with worsening symptoms over the past 3 days, including increased shortness of breath, wheeze, and cough, particularly at night. Her peak expiratory flow rate (PEFR) is 60% of her personal best. She is using her salbutamol inhaler every 2-3 hours with limited relief. According to BTS/SIGN guidelines, what is the most appropriate immediate management?