Applying for F type visa

Hi. I have organised to do a 3 month volunteer placement in china and have been told by the charity, who take on international volunteers all year round, that I need an F type entry permit. However on both occasions of trying to get this I have been told I need to apply for a working (Z) type even though I insist that I will mot be employed or paid. Can anyone give me advice with this? Im a UK citizen applying for a Chinese entry permit in Thailand. Thanks.