I would like apply for my first passport I was born here my

Hi I would like apply for my first passport I was born here my mother got indefinite remain to leave after I was I born and my father is trying had his remain but does not have the letter and attaching his passport to the application was not enough since the law set in 1983 needs to show proof off settlement what can we do?
JA: What steps have you taken? Have you filed any paperwork with the UK government? What country do you live in? What is your citizenship?
Customer: I live in the United Kingdom and applied for first passport British citizen
JA: Have you talked to a lawyer about this?
Customer: We have been to several we did a application through them but was sent a letter that we need to prove settlement of my father but he does not have the letter
JA: Is there anything else the Lawyer should know before I connect you? Rest assured that they'll be able to help you.
Customer: No Thankyou