We are in a similar situation OP.
We have an outstanding electrical certificate and our builder isn't responding to any communication from us. We paid our final bill and 'poof' he disappeared.
Eventually I managed to get in touch with the electrician who had done all of our work. He works as a subcontractor for the builder, and we didn't choose him or have any communication directly with him before now.
Luckily the electrician has been honest and told me he has been struck off the register of his official body due to having no fixed address due to a divorce.
He was registered when he began the work in Feb 2022, but was struck off in June 2022 and carried on working on our property (and others) until it was finished in November 2022.
The electrician has showed me emails from the builder proving that the builder was fully aware of the situation and told the electrician to just carry on and it would be ok.
So both the builder and electrician share the blame as far as I am concerned.
However, unfortunately according to our building inspector it is our own responsibility to make sure our electrical work can be certified. He accepts that the builder has been reckless but seems powerless to do anything about it, and has been clear that technically it is down to us to sort this.
I have discovered that it is illegal for another electrician to certify someone else's work on a part P certificate, unless they have supervised the work from the beginning. It seems that some electricians do this under the radar but it is risky.
Eventually our building inspector has agreed to allow us to get a full electrical survey done in lieu of a part P certificate. If the survey is ok then he will issue us with a completion certificate.
Our building inspector knows the original electrician and is confident that his work is sound- I think this partly influenced his decision to allow the electrical survey as an alternative option.
I am really angry with our builder. He was expensive but we thought he was reliable and capable but we had countless awful issues which I won't bore you with. Sadly he appears to have removed the 'review' section of his Facebook page so I can't warn anyone else about him.
I hope that helps a bit OP.