It appears that Villiers was British Ambassador from 1808 (arriving in December) to February 1810 when he was succeeded by Charles Stuart.
See page 101
See also Fortecue History of the British Army volume vii page 432.
You can find details of Villiers political life in
The House of Commons, 1790-1820, Volume 4
History of Parliament
The House of Commons, 1790-1820, R. G. Thorne, ISBN 0436521016, 9780436521010
Boydell & Brewer, 1986
List of Ministers to Portugal:
The Foreign office list and diplomatic and consular year book. Harrison and sons., 1857
John Villiers became the Earl of Clarendon.
Charles Stuart became Lord Stuart de Rothsay.