本人在16歳時英國讀書，5年後左右並且得到英國passport, 現在我已經44歲了，想申請4歲小孩英國passport, 英國HM要我提交 original Registration certificate，我也不知道什麼文件，想問有否師兄師姐知道嗎？ 謝謝你們🙏?

It is an official document you have received when you acquire the British citizenship. It is also the document you need when you first applied your British Passport.

There are 2 kinds of British citizenship - by descent or otherwise by descent. British citizenship can be only passed to children when it is otherwise by descent. HMPO requires the Registration certificate in order to determine what kind of British citizenship you have.

See here for detail:

https://www.gov.uk/get-replacement-citizenship-certificate