As JBW said.
The only requirement for "signing off" aka issuing an EIC or MEIC is that you are competent.
Are you, and can you demonstrate competency:
1, as the designer - has the installation been designed in all respects in complete compliance with the latest standards (18th+) , also 2, has it been installed in all respects in complete compliance with the latest standards? And finally 3, has it been fully inspected and tested in all respects in complete compliance with the latest standards?
I would suggest this is difficult to prove without knowledge/certification to the latest edition of the regulations, similarly without some testing certification it would also be difficult to demonstrate your competency.
Then, although a different side of things, part p demands proper notice to the local authority, this would normally be expected to be undertaken by you, so either membership of a scam, or direct acceptance from the authority, both of which would require proof of competency via certificates you don't yet have.