An Island of Europe
Britain is part of the continent of Europe. More precisely, Great Britain is a large island lying just off the northwestern coast of the European mainland. Although it is separated from the rest of the continent by water, it is firmly considered a European island, and the countries on it — England, Scotland, and Wales — are European nations.
Separated by a Narrow Sea
The body of water between Britain and mainland Europe is the English Channel. At its narrowest point, only a short stretch of sea separates the island from France — close enough that on a clear day, one coast can be seen from the other. Today the two are even physically linked beneath the waves by the Channel Tunnel, an undersea rail tunnel connecting Britain directly to the European mainland.
Once Joined to the Mainland
Britain has not always been an island. Long ago, during the last ice age, so much of the world's water was locked up in ice that sea levels were far lower. At that time, Britain was connected to mainland Europe by a stretch of dry land. People and animals could simply walk across. When the ice melted and sea levels rose, around nine thousand years ago, the rising water flooded that land bridge and turned Britain into the island we know today.
Source
This article was written using information from Wikipedia.