The name United Kingdom can only be used for dates from the year 1701.
For dates before 1701 you have to use England, Scotland or Wales.
The Acts of Union 1800, united the Kingdom of Great Britain and the Kingdom of Ireland in 1801, and introduced the name United Kingdom of Great Britain and Ireland.
The name "United Kingdom of Great Britain and Northern Ireland" was adopted in 1927 by the Royal and Parliamentary Titles Act. It reflected the reality of the independence of the Irish Free State, and the partitioning of Ireland in 1922, which left Northern Ireland as the only part of the island of Ireland still within the UK.
<http://en.wikipedia.org/wiki/United_Kingdom>