I am British national currently living in HK, what documents I need for marriage with Chinese girl?

I think it's easier for you to register your marriage at the local Civil Affairs' Bureau in your fiancee area. First, you should prepare a single certificate (also called Affidavit of Single Status, Certificate of No Record of Marriage, Certificate of No Impediment, Single Status Statutory Declaration, Certificate of Legal Capacity to Contract Marriage, etc). You should get it attested by your Ministry of Foreign Affairs and then authenticated by Chinese embassy in your country. Second, all your English documents should be translated by official translation companies. Then you can bring your single certificate, passport, your wife's Hukou (household registration book), both your birth certificates and official style photos (usually taken at the civil affairs bureau) for registration. Of course you will be asked to pay the handling fee.