It didn't. The Allies were victorious.
Great Britain, or more correctly The United Kingdom of Great Britain and Northern Ireland, was involved in both World War 1 and World War 2.
Britain didn't lose in World war 2. With some timely help from the US, they ended up on the winning side.
world war 1
There were many countries that were controlled by Great Britain at the beginning of World War 1.
The whole war
In 1940 the US France had been defeated, and only Great Britain stood in the way of a Nazi Western Europe. It was not certain that Great Britain would win or lose. If Great Britain fell, the US would be the only major democracy in the world.
Britain
Great Brittan did not want to lose the colonies it had claimed.
George VI was king of Great Britain during World War 2.
World War 2 was of two alliances. The Axis, and the Allies. Great Britain and U.S.A were on the Allies side.
Britain had a coalition government during World War 2
Great Britain declared war on Germany in September 1939 after Germany invaded Poland.