After the passing of his mother, Queen Elizabeth II not long after her platinum jubilee, Prince Charles has now acceded the throne as King Charles III in the UK.

Technically, he is the first king Charles of the UK. The kingdoms of England and Wales merged in 1535-42. But England (plus Wales) didn't merge with Scotland until much later, in 1707. However, both Charles I (1625-1649) and Charles II (1660-1685) styled themselves as kings of England (plus Wales), Scotland and Ireland. So the next number is naturally III?  

Well, the United Kingdom of Great Britain and Ireland was formed afterwards, in 1801 when it included the whole of Ireland. It was later renamed in 1927 as the United Kingdom of Great Britain and Northern Ireland following the establishment of the Irish Free State in 1922.

In any event, this was all resolved in 1953. John MacCormick and Ian Hamilton contested the right of the Queen to call herself "Elizabeth II" in Scotland. They claimed it was a breach of the Treaty of Union 1707 between England and Scotland, since Elizabeth I had been Queen of England but not of Scotland.

The court noted that MacCormick and Hamilton had not brought the claim out of criticism of the Queen or disloyalty, but merely regarding symbolism. The court ruled that there had been no breach of the Treaty of Union, they had no jurisdiction to interfere and essentially the Queen could use whatever title she wished. This was then confirmed upon the creation of the Scottish Parliament in 1999.

So King Charles III it is then. And, if you say he's not your king, the police might lead you away or you might even be arrested. Long live the King!