A 68-year-old woman presents with a 2-day history of productive cough, fever, and right-sided pleuritic chest pain. Her respiratory rate is 24/min, oxygen saturation is 93% on air, heart rate is 105/min, and temperature is 38.2°C. Chest auscultation reveals crackles in the right lower zone. Her CURB-65 score is calculated as 1 (age ≥65). According to NICE guidelines, what is the most appropriate initial antibiotic choice assuming no penicillin allergy?