A 60-year-old woman presents with a 2-year history of worsening bilateral knee pain, primarily on weight-bearing and relieved by rest. Stiffness is worst after inactivity but improves within 15-20 minutes of moving. Examination reveals bony enlargement of the knees, crepitus on movement, and mild varus deformity. X-rays show joint space narrowing, osteophytes, and subchondral sclerosis. What is the most appropriate first-line pharmacological management according to NICE guidelines?