You find an electrician and ask him to do a Periodic Inspection - now that it's all finished, and was done by someone unqualified, you won't get any other form of certificate from him. And presumably the person who did the work didn't issue an EIC either, so there will now never be one for this rewire.
The electrician doesn't need to be registered with a Part P scheme, because he can't certify the work as compliant with the Building Regulations, so the installation won't have the Completion Certificate it should have, and which could well be needed when the property is sold.
You might be able to apply to your LABC for regularisation, but they might insist that all the work is uncovered for inspection purposes.
Who organised the building works? How did they come to sort out only some of the Buildings Regulations approvals that were needed?