What length of visa can I get as a foreigner in a third country?

I am a UK citizen trying to apply for a chinese tourist visa in Bangkok. Ideally, I would like a 6 month multi-entry, but they have told me at the visa office that I can only apply for a 30 day. If I went to Hong Kong and applied from there, would this still be the case? Can I only apply for longer-term tourist visas from my home country?