I believe the so called rule change was 2006.
However, our daughter was born in March 2002. I and her mother were not married. We married 31st May 2006.
Daughter's passport application was not an issue. No extra documents, no Home Office involvement.
You'll find, if parents subsequently married, thus legitimising the birth, citizenship will be issued so long as all other criteria is met.
We are going off topic. I'll send you a PM.