Andrew Boyd, from Release International, also said that the "world is watching" to see whether Mr Cameron and Mr Osborne would challenge President Xi Jinping on his government's oppression of Christians, which includes their imprisonment and torture, the destruction of thousands of church crosses, and the complete demolishing of some church buildings.
In the case of church crosses, the Chinese government has said it's removed them for safety reasons.
Churches in China must receive state permission to function, leading many of the country's Christians to worship underground.
President Xi Jinping's visit to Britain is the first from the world's second largest economy and biggest exporter for ten years, and will also be an opportunity for the UK to encourage more investment into the country.
Andrew Boyd told Premier's News Hour: "David Cameron and George Osborne here have a golden opportunity to influence China for good.
"They can hear a call for religious freedom at a time when a president of China will be listening, and the world is watching to see that Mr Cameron and Mr Osborne use that opportunity well.
"Right now, one of China's most prominent Christian human rights lawyer [Zhang Kai] is being held illegally in a black jail - that's an unregistered detention centre - on possible spying charges, and being denied any legal representation.
"The crime that he's supposed to have committed is offering legal advice to churches whose crosses are being torn down by the thousand as China continues to persecute it's Christian minority.
"So the one question for David Cameron when he meets President Xi Jinping must be: when will you release Zhang Kai?"
Listen to Premier's Marcus Jones speaking to Andrew Boyd on the News Hour: