A 19-year-old female presents with a 2-day history of dysuria, urinary frequency, and suprapubic discomfort. She is otherwise well and has no fever or flank pain. A urine dipstick shows nitrites positive and leukocytes 2+. According to NICE NG109, what is the most appropriate antibiotic treatment for this uncomplicated lower urinary tract infection (UTI) in a non-pregnant woman?