A 42-year-old woman with a history of migraine without aura presents with recurrent headaches. She describes them as unilateral, throbbing, lasting 6-8 hours, associated with nausea and photophobia. She experiences these headaches 5-6 times per month, significantly impacting her work. She has tried simple analgesics with little effect. According to NICE CG150, what is an appropriate first-line prophylactic medication to consider?