@kaystar89 I understand the application is for the first passport as a person claiming to be British citizen based on the nationality of their ancestor(s).
Short birth certificate doesn’t show parents names so it was rightly rejected.
Grandparents are on the system for their own passports, right, but no one usually submit their marriage certificate for their own passport so the HMPO was again right to ask for proof of marriage of the ancestors as an identity doc for a citizen of a country is being issued for the first time. The applicant obviously must prove their entitlement to the passport. This, however, won’t happen again for renewals. Hmrc etc also do not have a copy of marriage certificate.
Accept my apologies if this forum wasn’t intended to be about things other than applications during lockdown.