A 4-year-year-old boy presents with a 2-day history of fever, sore throat, and refusal to eat. On examination, he has enlarged tonsils with exudate, cervical lymphadenopathy, and a temperature of 38.8°C. A rapid antigen test for Group A Streptococcus is positive. He has no history of penicillin allergy. According to NICE/PHE guidelines, what is the most appropriate antibiotic treatment?