The British people, or the Britons, are the citizens of the United Kingdom of Great Britain and Northern Ireland, the British Overseas Territories, and the Crown dependencies. British nationality law governs modern British citizenship and nationality, which can be acquired, for instance, by descent from British nationals. When used in a historical context, "British" or "Britons" can refer to the Celtic Britons, the indigenous inhabitants of Great Britain and Brittany, whose surviving members are the modern Welsh people, Cornish people and Bretons. Although early assertions of being British date from the Late Middle Ages, the creation of the united Kingdom of Great Britain in 1707 triggered a sense of British national identity.
Union and the development of Britishness
Despite centuries of military and religious conflict, the Kingdoms of England and Scotland had been "drawing increasingly together" since the Protestant Reformation of the 16th century and the Union of the Crowns in 1603. A broadly shared language, island, monarch, religion and Bible (the Authorized King James Version) further contributed to a growing cultural alliance between the two sovereign realms and their peoples. The Glorious Revolution of 1688 resulted in a pair of Acts of the English and Scottish legislatures--the Bill of Rights 1689 and Claim of Right Act 1689 respectively--which ensured that the shared constitutional monarchy of England and Scotland was held only by Protestants. Despite this, although popular with the monarchy and much of the aristocracy, attempts to unite the two states by Acts of Parliament in 1606, 1667, and 1689 were unsuccessful; increased political management of Scottish affairs from England had led to "criticism", and strained Anglo-Scottish relations.  While English maritime explorations during the Age of Discovery gave new-found imperial power and wealth to the English and Welsh at the end of the 17th century, Scotland suffered from a long-standing weak economy. In response, the Scottish kingdom, in opposition to William II of Scotland (III of England), commenced the Darien Scheme, an attempt to establish a Scottish imperial outlet--the colony of New Caledonia--on the isthmus of Panama. However, through a combination of disease, Spanish hostility, Scottish mismanagement and opposition to the scheme by the East India Company and the English government (who did not want to provoke the Spanish into war) this imperial venture ended in "catastrophic failure" with an estimated "25% of Scotland's total liquid capital" lost.  The events of the Darien Scheme, and the passing by the English Parliament of the Act of Settlement 1701 asserting the right to choose the order of succession for English, Scottish and Irish thrones, escalated political hostilities between England and Scotland, and neutralised calls for a united British people. The Parliament of Scotland responded by passing the Act of Security 1704, allowing it to appoint a different monarch to succeed to the Scottish crown from that of England, if it so wished. The English political perspective was that the appointment of a Jacobite monarchy in Scotland opened up the possibility of a Franco-Scottish military conquest of England during the Second Hundred Years' War and War of the Spanish Succession. The Parliament of England passed the Alien Act 1705, which provided that Scottish nationals in England were to be treated as aliens and estates held by Scots would be treated as alien property, whilst also restricting the import of Scottish products into England and its colonies (about half of Scotland's trade). However, the Act contained a provision that it would be suspended if the Parliament of Scotland entered into negotiations regarding the creation of a unified Parliament of Great Britain, which in turn would refund Scottish financial losses on the Darien Scheme.

Q: Who became united?
A: the Kingdoms of England and Scotland
What led to their union?
drawing increasingly together" since the Protestant Reformation of the 16th century and the Union of the Crowns in 1603.
drawing increasingly together" since the Protestant Reformation of the 16th century and the Union of the Crowns in 1603.