The British Embassy in China is not authorised to perform marriages ceremonies. Marriages in China performed in accordance with Chinese law are generally recognised in UK law. Marrying in Mainland China, require a Certificate of No Impediment (CNI), a document which shows there are no objections to your marriage.
Once a British National is married, he can also deposit his Marriage Certificate with the General Records Office (GRO) in the UK but it is not a requirement in UK law.
Please note that in UK law, a foreign national does not automatically acquire British nationality through marriage to a British national. If you marry a Chinese national, they will require a visa to visit or settle in the UK. English language ability requirements may also apply. Detailed information on how to apply for a visa or settlement can be obtained from the UK Borders Agency.