Head of the Armed Forces is the position of the sovereign of the United Kingdom as commander-in-chief of the British Armed Forces. Supreme military authority is vested in the monarch and extends to the exercise of several personal prerogatives. However, routine administration of the military is delegated as a matter of law to the Defence Council of the United Kingdom, a body officially charged with the direction and command of the Armed Forces. As the Defence Council and its service boards are all a part of the Ministry of Defence, which itself is a ministerial department of the Government of the United Kingdom, the prime minister makes the key decisions on the use of the Armed Forces, while the secretary of state for defence assists the prime minister in the development of defence policy and administers the day-to-day military operations.

Before joining the military all recruits of the British Armed Forces (other than Officers in the Royal Navy) must take the following oath:

I... swear by Almighty God (do solemnly, and truly declare and affirm) that I will be faithful and bear true allegiance to His Majesty King Charles III, His Heirs and Successors, and that I will, as in duty bound, honestly and faithfully defend His Majesty, His Heirs and Successors, in Person, Crown and Dignity against all enemies, and will observe and obey all orders of His Majesty, His Heirs and Successors, and of the [Generals/Admirals/Air Officers] and Officers set over me.

Supreme command and control over the Armed Forces vests in the Sovereign by virtue of the royal prerogative; this constitutional arrangement has existed in the United Kingdom and its successor states since time immemorial. As a matter of statute law however, day-to-day administration and control of the military is carried out on behalf of the reigning King or Queen by the Defence Council through the Ministry of Defence and its subordinate service boards. This delegation of command authority extends to matters of mobilisation, the recruitment of non-commissioned officers, and the issuance of commands through the command hierarchy of the three services. That notwithstanding, the delegation of daily command authority does not in any way affect the Crown's personal prerogatives in relation to the Armed Forces.

A number of powers, duties, and functions are reposed in the Sovereign in his or her capacity as Head of the Armed Forces. Long-standing constitutional convention, however, generally requires the Sovereign to exercise these personal prerogatives in accordance with the advice of ministers of the Crown responsible to Parliament, namely the Prime Minister and His Majesty's Secretary of State for Defence. The Prime Minister (in consultation with the Cabinet) makes the key political decisions on the use of the Armed Forces while the Defence Secretary administers daily military operations. Nevertheless, the supreme military authority belongs to the Sovereign and not to the Government of the day. The Sovereign thus enjoys the right to be consulted before acting on ministerial advice, the right to encourage a given course of military action, and the right to warn against or request a reconsideration of the same. In addition, the monarch has a number of reserve powers at his disposal, albeit rarely invoked, to uphold responsible government and prevent unlawful use of the Armed Forces. To this end, the King has plenary authority to:

The King regularly receives the agenda in advance of all Cabinet meetings and its committees, particularly the National Security Council. He also receives the minutes of the meetings of all of these committees and all Cabinet documents. Likewise, the King regularly receives the Secretary of State for Defence in audience to discuss defence documents and policies; if he requires, his Private Secretary can seek additional information from the Ministry of Defence. His Majesty also receives regular reports from the Chief of the General Staff, the First Sea Lord and Chief of the Naval Staff, and the Chief of the Air Staff that cover all important developments in military matters. His Majesty further receives the Chief of the Defence Staff in audience to discuss tri-service military matters.

Since the reign of Elizabeth II, the Monarch has been affectionately referred to as The Boss in informal settings by some members of the Armed Forces, reflecting the Monarch's connection to the Armed Forces through their position as Commander-in-Chief and the Oath service personal take when joining the Armed Forces.

The Defence Services Secretary is an officer of the Royal Household. The DSS is appointed by Royal Warrant from the three Services on a rotational basis. The current occupant is Rear Admiral James Norman Macleod. He is the primary channel of communication between the Monarch, in his capacity as Commander-in-Chief of the Armed Forces, and the officers and commands of the British Armed Forces. His office provides the Monarch with the information and documentation related to defence affairs that he requires to perform his military functions. The office was created in 1964 by Queen Elizabeth II as part of the centralization of military affairs into a single defence ministry. The DSS maintains a direct link with the offices of the chiefs of staff of the Army, Navy and Air Force. The Defence Services Secretary is responsible to the King, the Secretary of State for Defence and the Chief of the Defence Staff for tri-service military appointments and works with the Military Secretary, the Air Secretary and Naval Secretary. He submits the names of the officers nominated for promotion to the King through his private secretary. The King makes appointments to the ranks of rear-admiral, major-general, air vice-marshal and above directly.