A 6-month-old infant presents with a 3-day history of coryza and low-grade fever, followed by the onset of a harsh, barking cough, hoarseness, and inspiratory stridor, particularly when upset. On examination, the child is alert, has mild intercostal recession, respiratory rate of 40/min, and audible stridor at rest. Oxygen saturation is 95% in air. What is the most appropriate management according to UK guidelines?