There seems to be increasingly anti-monarchist sentiment in the UK, and although I don't see this happening any time soon I think it's worthwhile asking how this would ever work.
How would this ever work? Could parliament present a bill? Could there be a referendum and would the current British monarchy have to accept the result or decline as ultimate head of state?
Several countries have already transitioned from having the British monarch as head of state, to being a republic.
Many former British colonies became independent nations with the King or Queen of the UK as their head of state. Some, such as Canada, Australia, and New Zealand, remain constitutional monarchies today; but others have become republics. Examples include:
Ireland: Independent 1922, republic 1937.
India: Independent 1947, republic 1950.
South Africa: Independent 1910, republic 1961.
All of the above countries had legal and political systems very closely based on the British model. As it turned out, there was no barrier to a peaceful and lawful transition from constitutional monarchy to republic. Another relevant example is:
These precedents indicate that, if there was sufficient popular demand in the UK, a way would be found to legally abolish the monarchy. It would be implemented by one or more Acts of Parliament, and most likely authorised by a referendum as in the Australian example.
This does not address whether such a thing is likely to happen; the monarchy is central to British national identity, and current opinion polling puts support for a republic in the UK at about 19%.
Any transition from constitutional monarchy to republic raises questions: How will the head of state be chosen, what powers will he or she have, whose face will go on the coins and postage stamps, and so on.
The UK becoming a republic would raise some unique additional questions:
Would any Commonwealth realms choose to remain monarchies? If so, would the royal family relocate to one of them?
What would be done with the extensive royal estates in the UK? How much would remain the private property of the erstwhile royals, and would they be subject to inheritance tax? This includes not just land and buildings, but also the largest private art collection in the world.
Would the former monarch remain as Supreme Governor of the Church of England?
Would the Privy Council continue to exist? Would its membership and functions be reformed?
However, all of the above could be settled through existing channels for decision-making.
The Constitutional framework of the UK is a combination of laws, traditions and conventions. The central principle of the constitution is "Parliamentary Sovereignty". It was established, by the deposition of James II that Parliament has the power to remove a monarch, and to choose a successor. It is, therefore, reasonable to believe that Parliament also has the right to end the monarchy. A simple Act of Parliament would be sufficient.
Such an act of parliament would formally need royal assent. However the convention is that the Queen acts only on the advice of her Ministers, and she could not personally choose to not to sign. As JBentley points out the requirement for the monarch not to refuse royal assent is a convention and as such not legally enforceable. Conventions do evolve over time with old ones being ignored, and new ones coming into effect. A convention is only as strong as the will of the political actors to abide by it.
A referendum could be called by Parliament. Major constitutional changes are often decided by referendum, and so this becomes a tradition.
There would be a lot of loose ends. For example, would an elected President take the role of the Monarch? This would be a simple arrangement, as the various reserve powers of the Crown could be passed to the President. If there is to be no replacement then things like the oath of loyalty that soldiers swear would have to be changed.
There would also be the large amount of property that the Crown owns. The act would need to establish what is owned by the Queen, and what is owned by the Country.
The example of James II is that the King was not able to prevent the will of Parliament. However this was ultimately an act of force. William was able, with the support of Parliament, to take the Throne, and James was unable to raise a sufficient army to prevent it. If the monarch chose to reject the decision of Parliament, then there would be a de facto state of civil war.
The final decider of what is constitutional is the question of who can defend their position by force. If a monarch has sufficient support, especially in the armed forces then they don't need to agree to anything.
The relevant principle of law in the United Kingdom is parliamentary sovereignty. Basically, parliament may pass or repeal any law (ignoring exceptions, like treaties). So Parliament would have to choose to end the monarchy.
It's possible that parliament might choose to end the monarchy by passing a law enabling a "binding" referendum that would allow people to vote on the issue. The point is that they could repeal that law at any time before it took effect. And only parliament has the power to call a referendum unless parliament delegates that power to someone else.
Ultimately, an end to the monarchy could only occur with the consent of parliament. Under some circumstances, it might be possible for that consent to be implicit.
