The Answer:

The United Kingdom of Great Britain and Northern Ireland encompasses the most of the three. As its full name suggests, it includes Northern Ireland in addition to Great Britain.

Great Britain is composed of three areas: England, Scotland, and Wales.

As for England, it is the largest and most populous portion of the United Kingdom.

In practice, the terms are often used interchangably through metonymy, though those living in the UK but outside of England tend not to be fond of the practice.

—The Editors