Please note that there is no British Embassy in Tonga and you will have to obtain advice from the British High Commission in Fiji. Note the emergency telephone number below if you need assistance after being arrested or after being attacked or involved in an accident or the victim of a crime. Note that they do not handle visa inquiries.
If you are in Tonga and you urgently need consular assistance (eg if you’ve been attacked, arrested or there has been a death), call +679 3304766 on the 24-hour helpline number.
British Birth Abroad:
You need to do this yourself as well. This needs to be done by post. See here which documents need to be completed and where you will need to send the documents.