A dukedom or a princedom (principality) does not become a Kingdom unless there is a king involved.
For Brunswick, read: "Braunschweig":
Ernest Augustus of Brunswick-L�neburg had married Sophie granddaughter of the Scottish king James I of Great Britain. The British Act of Settlement (1701) designated Sophie heiress of the British crown after Queen Anne, but because Sophie died shortly before Anne in 1714, her son Georg Ludwig succeeded as George I of GB (1714-1727). He was the first of five monarchs of the House of Hanover and he ruled over both Hanover and Great Britain. He assumed the title of King of Hanover on 12 August 1814 following negotiations at the Congress of Vienna. Hanover was expanded to the North Sea by the addition of Bremen and Verden in 1715 and the bishopric of Osnabruck in 1803.
Hanover remained united to the British Crown until 1837 when King William IV died and Queen Victoria succeeded to the British throne. As women were barred from the Hanoverian succession King William IV was succeeded in that country by his brother Ernst August, Duke of Cumberland (1771-1851).
The last King of Hanover was George V (1819-1878) who was deposed on 20 September 1866 when Prussia annexed Hanover having taken the side of Austria in their war with Prussia over the administration of Schleswig-Holstein.
After the death of King George V his son Prince Ernst August (1845-1923) took the title of Duke of Cumberland, Duke of Brunswick and Luneburg on 11 July 1878.