LONDON — To the recorded peals of Big Ben and the gentle fluttering of Union Jacks, Britain bade farewell to the European Union at 11 p.m. on Friday, severing ties to the world’s largest trading bloc after nearly half a century and embarking on an uncertain future as a midsize economy off the coast of Europe.

For Britain, having transitioned in the postwar era from a globe-girdling empire to a reluctant member of the European project, it was yet another epoch-making departure.

It is a departure that will upend settled relations in virtually all areas of society, the economy and security matters, while confronting Britain with new questions of national identity. Three and a half years after the former prime minister, Theresa May, proclaimed that “Brexit means Brexit,” the British government will finally have to decide precisely what that means.

Britain must still negotiate its future trade relations with the European Union, a thorny process that could take through the end of the year, or longer.