A 30-year-old man presents to the Emergency Department with palpitations and light-headedness that started suddenly 30 minutes ago. His ECG shows a regular, narrow complex tachycardia at a rate of 170 bpm. P waves are not clearly visible. His blood pressure is 115/75 mmHg and he appears clinically stable. Vagal manoeuvres (Valsalva) have been attempted unsuccessfully. What is the most appropriate next pharmacological step according to Resuscitation Council UK guidelines?