A 40-year-old man with type 1 diabetes mellitus presents to the Emergency Department feeling unwell. He has vomited twice. His capillary blood glucose is 22 mmol/L, and capillary ketones are 4.5 mmol/L. His pH on venous blood gas is 7.20, and bicarbonate is 14 mmol/L. What is the most critical initial management step according to UK guidelines?