THE British team leader of the Sino-British Joint Liaison Group (JLG), Anthony Galsworthy, will soon take up a job in the British Cabinet Office. The UK Government yesterday announced Mr Galsworthy would leave his post in early autumn and Hugh Davies would succeed him. An ''old China hand'' on Sino-British relations, Mr Galsworthy will not be completely cut off from Hongkong matters in his new job. His responsibility as an undersecretary of state will be foreign policy. Mr Galsworthy took up the JLG job in November 1989, only a few months after the June 4 Beijing massacre. Mr Davies is temporary Senior British Trade Commissioner in Hongkong - a post which was vacated by Stephen Day for health reasons. He will hand over the trade post to Francis Cornish, head of the Foreign Office News Department, in mid-September. Mr Davies has spent much of his diplomatic service career involved with Chinese affairs and his first posting was in the British Embassy in Beijing. From 1990 until earlier this year, he was the head of the Far Eastern Department in the Foreign Office with responsibility for Britain's relations with China and northeast Asia generally.