A 19-year-old student presents to the Emergency Department confused and drowsy. Her friends report she has type 1 diabetes and has been feeling unwell with vomiting for the past day. On examination, she is tachycardic (120/min), tachypnoeic (Kussmaul breathing), and dehydrated. Her blood glucose is 28 mmol/L and capillary ketones are 4.5 mmol/L. Venous blood gas shows pH 7.15. According to UK guidelines, what is the most critical initial intravenous fluid management?