He called himself "king of Great Britain" after gaining England's throne in 1603--he was already king of Scotland
... the second 
son of James VI of Scotland (from 1603 also James I of England) and Anne ... 
Charles found himself ever more in disagreement on religious and financial 
matters... [DOC] [TLE] Family of James Stuart - James I of England and Anne of Oldenburg [PAR] Note: James VI and I (19 June 1566  27 March 1625) was King of Scotland as ... 
the union of the Scottish and English crowns on 24 March 1603 until his death.