A 29-year-old woman presents with heavy menstrual bleeding, reporting soaking through pads every 1-2 hours during the heaviest days, causing significant disruption to her life. She is otherwise well and examination is unremarkable. Full blood count shows Hb 105 g/L, MCV 78 fL. What is the most appropriate first-line pharmacological treatment according to NICE guidelines, assuming no contraindications and contraception is not desired?