Non-US (English-speaking) writers, do you write in American English?

A novelist friend of mine from the UK insists on writing her novels in American English (color instead of colour, double quotation marks instead of single, etc), because she says it's now a universally recognised language.

My question is, particularly to British writers, do you stick to British English spelling and punctuation rules, or adopt American English?

Is it a recognised practice to do so? Or should I stick to British English and single quotation marks for dialogue and buck this trend?