All landmasses on Earth are constantly shifting and rearranging. The British Isles most likely just broke off as all landmasses do at one point or another. However, during the Ice Age (for example), the British Isles and Europe were connected due to the low sea levels. But once the ice melted, sea levels rose and the land bridge disappeared.
because of the politic and the economy
Poland, Czechoslovakia, Hungary, and Yugoslavia separated the Soviet Union from Western Europe.
The Atlantic Ocean separated the USA from the war in Europe.
Europe and Asia
Europe and Asia are on the same land mass, not separated land masses, so therefore these cannot be continents. A continent is a body of land separated by water. Europe nor Asia are separated by water.
Switzerland is a country that is separated from the rest of Europe by the Alps mountain range.
Europe is not in North America. Europe and North America is separated by the vastness of the Atlantic Ocean.
Asia
America and Europe
The Pyrenees Mountains have kept Spain separated from Europe for much of history. They're on the border of France and Spain.
Eastern North America and Western Europe are separated by the Atlanic Ocean. Northern Europe and the northern part of North America are separated by the Arctic Ocean.
Russia Turkey Georgia Azerbaijan Kazakhstan
Northern Canada and Europe are separated by the ocean.