Missing building control and completion certificates on conversion remortgage?

Hi all, I’m currently in the process of remortgaging a leasehold flat and have run into an issue following the lender’s valuation.

The property is an office-to-residential conversion (completed around 10 years ago) in a block of flats. The valuer has down-valued the property to £0, stating that they require a Structural Engineer or Building Surveyor’s report from the developer confirming the property’s suitability for residential conversion with regard to structure and condition

I have already checked with:

Managing agent
Freeholder
None of them holds any Building Regulations Completion Certificate, Final Certificate, or any structural sign-off documentation.

The original developer is now dissolved and cannot be contacted. There was only a new build warranty in place, and I am currently contacting the warranty provider and the local authority building control.

My questions:
Has anyone encountered a similar situation with missing building control/completion certificates on conversions?
If no certificate can be located, what is the best practical route forward?

Any advice or similar experiences would be greatly appreciated.

Thanks in advance.

Phoebe