A 78-year-old man is admitted with confusion, fever, and cough productive of purulent sputum. Chest X-ray confirms right lower lobe consolidation. His CURB-65 score is calculated as 3 (Confusion=1, Urea >7 mmol/L=1, RR $\ge$30/min=0, BP<90/60=0, Age $\ge$65=1). According to NICE guidelines, what is the most appropriate initial antibiotic regimen?