when I asked what policing district he was posted, he didn't answer.

A good guess then was that he was a cop under the HKPD politico dept(not under any regional district) which was disbanded in 1995. Seems members of that dept were given UK passports. Later he admitted he really was a politico dept cop.

Then a while later, he said he was posting from the States. Either he moved there or he wanted to work for the yanks.

(Sure it doesn't mean he is necessarily born in HK...)