A 34-year-old man with a 2-day history of generalised urticaria after eating peanuts suddenly develops breathlessness, lip and tongue swelling, and feels faint. He is tachycardic at 122 bpm, blood pressure 80/50 mmHg, respiratory rate 30 breaths/min, and oxygen saturation 90% on air with widespread wheeze. What is the most appropriate immediate treatment?