A 65-year-old man is admitted with an acute exacerbation of chronic obstructive pulmonary disease (COPD). He is breathless at rest with increased sputum purulence. His oxygen saturation is 86% on room air. Arterial blood gas (ABG) on controlled oxygen (24% Venturi mask) shows: pH 7.30, PaCO₂ 8.5 kPa, PaO₂ 7.5 kPa, HCO₃⁻ 30 mmol/L. What is the most appropriate next step in respiratory support according to NICE guidelines?