The Guardian Style Guide (http://www.guardian.co.uk/styleguide/c
all lc, eg prime minister, US secretary of state, chief rabbi, editor of the Guardian. titles
cap up titles, but not job description, eg President Barack Obama (but the US president, Barack Obama, and Obama on subsequent mention); the Duke of Westminster (the duke at second mention); Pope Benedict XVI but the pope."
While The Economist (http://www.economist.com/style-guide/capitals
“All office-holders when referred to merely by their office, not by their name, are lower case: the chancellor of the exchequer, the foreign secretary, the prime minister, the speaker, the treasury secretary, the president of the United States, the chairman of Coca-Cola.
The only exceptions are
(1) a few titles that would look unduly peculiar without capitals, eg, Black Rod
, Master of the Rolls
, Chancellor of the Duchy of Lancaster
, Lord Privy Seal
, Lord Chancellor
(2) a few exalted people, such as the Dalai Lama
and the Aga Khan
. Also God
and the Prophet
Some titles serve as names, and therefore have initial capitals, though they also serve as descriptions: the Archbishop of Canterbury, the Emir of Kuwait, the Shah of Iran
. If you want to describe the office rather than the individual, use lower case: The next archbishop of Canterbury will be a woman. Since the demise of the ninth duke, there has never been another duke of Portland.
Hope that helps,