FAA if it’s going on their register.
https://www.faa.gov/aircraft/air_cer.../aw_cert_proc/
On smaller stuff in the U.K. the Certificate of Airworthiness is none expiring, but is only valid with an Airworthiness Review Certificate, so if the CAA hasn’t withdrawn the C of A for any reason, it would be down to me as a CAMO, I would inspect the aircraft and paperwork, deem what is required to put it back in the air, ensure the work is carried out or do it myself and then when satisfied issue an ARC to revalidate the C of A and send a copy in to the CAA.
You just have to be careful to not miss anything component life wise or AD’s etc.
What rank do you have to hold in the RAF these days to be a CAMO?
..