Answer by taking a quote from the following article:

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.

The indigenous people of the British Isles have a combination of Celtic, Norse, Anglo-Saxon and Norman ancestry.  Between the 8th and 11th centuries, "three major cultural divisions" had emerged in Great Britain: the English, the Scots and the Welsh, the earlier Brittonic Celtic polities in what are today England and Scotland having finally been absorbed into Anglo-Saxon England and Gaelic Scotland by the early 11th century. The English had been unified under a single nation state in 937 by King Athelstan of Wessex after the Battle of Brunanburh. Before then, the English (known then in Old English as the Anglecynn) were under the governance of independent Anglo-Saxon petty kingdoms which gradually coalesced into a Heptarchy of seven powerful states, the most powerful of which were Mercia and Wessex. Scottish historian and archaeologist Neil Oliver said that the Battle of Brunanburh would "define the shape of Britain into the modern era", it was a "showdown for two very different ethnic identities - a Norse Celtic alliance versus Anglo Saxon. It aimed to settle once and for all whether Britain would be controlled by a single imperial power or remain several separate independent kingdoms, a split in perceptions which is still very much with us today". However, historian Simon Schama suggested that it was Edward I of England who was solely "responsible for provoking the peoples of Britain into an awareness of their nationhood" in the 13th century. Scottish national identity, "a complex amalgam" of Gaelic, Brittonic, Pictish, Norsemen and Anglo-Norman origins, was not finally forged until the Wars of Scottish Independence against the Kingdom of England in the late 13th and early 14th centuries.  Though Wales was conquered by England, and its legal system replaced by that of the Kingdom of England under the Laws in Wales Acts 1535-1542, the Welsh endured as a nation distinct from the English, and to some degree the Cornish people, although conquered into England by the 11th century, also retained a distinct Brittonic identity and language. Later, with both an English Reformation and a Scottish Reformation, Edward VI of England, under the counsel of Edward Seymour, 1st Duke of Somerset, advocated a union with the Kingdom of Scotland, joining England, Wales, and Scotland in a united Protestant Great Britain. The Duke of Somerset supported the unification of the English, Welsh and Scots under the "indifferent old name of Britons" on the basis that their monarchies "both derived from a Pre-Roman British monarchy".  Following the death of Elizabeth I of England in 1603, the throne of England was inherited by James VI, King of Scots, so that the Kingdom of England and the Kingdom of Scotland were united in a personal union under James VI of Scotland and I of England, an event referred to as the Union of the Crowns. King James advocated full political union between England and Scotland, and on 20 October 1604 proclaimed his assumption of the style "King of Great Britain", though this title was rejected by both the Parliament of England and the Parliament of Scotland, and so had no basis in either English law or Scots law.

Where did the british people originate?
Scotland