In September 1956 just before the Suez Crisis. French Prime Minister Guy Mollet met up with British Prime Minister Anthony Eden in London. Mollet proposed a union between the United Kingdom and the French Union with Elizabeth II as head of state and a common citizenship. As an alternative, Mollet proposed that France join the Commonwealth. British Prime Minister Anthony Eden rejected both proposals and France went on to join the Treaty of Rome, which established the European Economic Community and strengthened the Franco-German cooperation.But what if Eden accepted Mollet's first offer and then a new rising superpower grew, The Franco-British Union was born.
As Britain and France decided to stick together, the Suez War ended in British/French\Israeli victory and then in early December 1956, the Governments of Great Britain, Ireland and France all met up in Cherbourg were it was decided that the Union will include Britain, Ireland and France as well as all their overseas colonies. The Treaty of Cherbourg was signed setting up the new country.
BBC News Report on the Union - []