A 62-year-old woman presents with a 3-month history of fatigue, weight loss, and intermittent upper abdominal pain. An upper GI endoscopy reveals a large, ulcerated gastric mass. Biopsies confirm gastric adenocarcinoma. CT scan shows localised disease with enlarged perigastric lymph nodes but no distant metastases. What is the most appropriate next step in management according to NICE guidelines?