Système politique britannique : monarchie constitutionnelle

In the context of the British political system, the monarch plays a specific role often defined as a 'Constitutional Monarchy'. Based on your knowledge of the UK's uncodified constitution, explain the following: 1) What is the fundamental difference between the 'Head of State' and the 'Head of Government'? 2) Why is it said that the monarch 'reigns but does not rule'? Provide a short analysis ($150$ to $200$ words) using key terms like 'Royal Assent', 'Neutrality', and 'Parliamentary Sovereignty'.