I think this depends on a lot of things.. Probably most obvious is the exportation of American media and American prescence throughout the world. However, this is also true to some degree with British English. What kind of English do you think the original British colonies spoke? Considering the United States didn't exist at the time, I'd assume it was British English. The influence of the mother country is a major factor. Second, I would say the availability of teachers and learning resources. As a teacher, I would never attempt to teach students in British English. Simply because I'm not British, and I'm not going to don a fake accent to try to teach them, as it would likely result in me teaching them wrong. Many schools, especially in countries struggling to get native speakers can't afford to have a preference towards British or American or Austrailian English. They'll take what they can get, and given the population differences it may be more likely to be American. Next, I would say similarity to their own native language. As I study Japanese, the people I've spoken to say they have a much easier time understanding British English. This is because American English is very direct and to the point and often has little natural respectful tone or consideration for the other's feelings. British English tends to be much softer in this way, which is more similar to the Japanese language, so they tend to be more comfortable speaking it. That being said, the vast amount of American media infiltration on Japan may be a much stronger influence towards American English. Lastly, there are cases where a person has a personal preference. Maybe because a close friend or something uses this accent or some kind of personal connection to it. Personally, I haven't seen this as a common thing, most just use what they've been taught in the past. Maybe as a British English teacher you run into it more often? I'm not sure.

Americanish is obviously getting more popular in China, even the English textbooks are inclined to teach American English, on the aspects of pronunciation, spelling, vocabulary, & usage. Maybe it's mainly because USA is stronger than UK in some respects, such as international status, economic mightiness, military power, & so on, but I still think that, when it comes to the topic of "English teaching", perhaps UK (British English) has much more authority, experience, & elegance than USA (American English). & British dictionaries also tend to involve corresponding American English, while American dictionaries almost list nothing about British English usages inside. & British dictionaries & grammar books are much more suitable for non-English speakers to learn many nuances or unintelligible problems, while American ones are only for English native speakers. Therefore, the brands such as Oxford, Longman, Cambridge, Collins, are all my favourite dictionaries, & I don't like American K.K. phonetic symbol so much, it even seems very strange and laughable, though a lot of Americans consider British English as an interesting accent, by contraries. So, I'll always support British English of UK, especially London or Oxford accent. Long live, British English.

Oh, and Richard (I think that's your name, right? Maybe I've forgotten :P) It's just as strange as an American to hear a foreigner speaking British English. I think this is probably just due to our amusement with Americans picking up fake British accents or vice-versa. When Americans spend time in England and come back to the US and speak with a British accent, they tend to be relentlessly mocked for using a 'fake' accent, and from what I've heard from a British the same situation is quite true from the British point of view. I guess it just seems odd for someone to learn your language in another accent, so that they have two different accents together (Chinese accent of American accent, for example).

Yes, I must admit that, US is, for sure, a highly developed country in terms of economy, weapon, science & technology, but it doesn't mean, at least not entirely mean that "American English is really superior to British English", & the reason, why so many people prefer Americanish, is mainly due to the proverb "love me,love my dog".（爱屋及乌）

I have no other choice, I live in the border I learned this one because I listen to it more