You're following rules to apply for British citizenship as a
child, you're no longer a child and them rules don't apply. Annoyingly reading
this document, and following the chart on page 11, you would have been entitled to register as a British citizen before you turned 18 by the living here for 10 years rule. Everything you're following with the parents rule etc is void, and that's simply because you're no longer defined as a child and the reason the passport may have been declined was because you had no certificate of UK registration or naturalisation.
Now as an adult the situation is a whole lot more difficult, if you look at the document on the bottom page you'll see a chart which leads to only one thing...that being applying for naturalisation.
There is contact information on that form, maybe you can get some help but they'll go the book and only by the book usually.