The head of state and theoretical source of executive, judicial and legislative power in the UK is the British monarch, currently Queen Elizabeth II. However, sovereignty in the UK no longer rests with the monarch, since the English Bill of Rights in 1689, which established the principle of Parliamentary Sovereignity. Despite this the Monarch remains Head of State, akin to a President in European (but not American) political tradition.
Queen Elizabeth II & Today the Sovereign has an essentially ceremonial role restricted in exercise of power by convention and public opinion. However the monarch does continue to exercise three essential rights: the right to be consulted, the right to advise and the right to warn . Prime ministers have weekly confidential meetings with the monarch.
Originally the monarch possessed the right to choose any British citizen to be her Prime Minister and could call and dissolve Parliament whenever he or she wished. However, in accordance with the current 'unwritten constitution', the Prime Minister is the leader of the largest party in the House of Commons and Parliament is dissolved at the time suggested by him or her. The monarch retains the ability to deny giving a bill Royal Assent, although in modern times this becomes increasingly more unlikely, as it would cause a constitutional crisis. Queen Anne was the last monarch to exercise this power, which she did on 11 March 1708 with regard to a bill "for the settling of Militia in Scotland". Other royal powers called royal prerogative, such as patronage to appoint ministers and the ability to declare war, are exercised by the Prime Minister and the Cabinet, with the formal consent of the Queen.
In formal terms, the Crown in Parliament is sovereign even though in practical terms the political head of the UK is the Prime Minister (Tony Blair since 2nd May, 1997). However, the real powers of position of the Monarch in the British Constitution should not be downplayed. The monarch does indeed retain some power, but it has to be used with discretion. She fulfils the necessary constitutional role as head of state, and with the absence of a distinct separation of powers in the American model and a strong second chamber, acts as a final check on executive power. If a time came to pass, for instance, when a law threatened the freedom or security of her subjects, the Queen could decline royal assent, free as she is from the eddies of party politics. Furthermore, armed removal of Parliament or Government would be difficult, as the Monarch remains commander-in-chief of the armed forces, who swear an oath of allegiance to her.