A 33-year-old man presents with a 2-week history of bloody diarrhoea, abdominal cramps, and urgency. He is opening his bowels 8-10 times per day. Flexible sigmoidoscopy reveals continuous inflammation with loss of vascular pattern, contact bleeding, and ulceration extending from the rectum to the sigmoid colon. Biopsies confirm active chronic colitis. What is the most appropriate initial treatment to induce remission according to NICE guidelines?