Each country has its own requirements with regards to who can notarise/certify documents. Here these are but not limited to certified public accountants, justices of the peace and lawyers. The British Consulate will certify documents but I am not sure whether this is restricted to documents issued by them. You could ring them on 2901-3000.
We are infact going tomorrow to the US consulate to notarize a few documents to be sent to USA.Our lawyers in USA adviced us that only the US consulate notarized documents will be accepted in USA.They give an appointment in 10 -15 days time.
The US consulate web address is http://hongkong.usconsulate.gov/
We had 4 sets of papers notarised and the whole process took 15 mins.There are dates available for next week online.