I understood that the airframes were certified and approved by A.R.B. for the issue of its Certificate of Airworthiness - but that the certificates were then issued by the Ministry of Civil Aviation.
The CAA is presently responsible for maintaining the the UK Register of Civil Aircraft. I am presuming that this function was carried out by the Ministry prior to the CAA's founding in 1972.
The following is the text of part the evidence given in the inquiry into the crash og G-ALYY which explains the procedure. I also include a link to the site, which is excellent.
"Yoke Yoke was granted a Certificate of Registration No. R.3221/1 on the 18th September, 1951 in the name of B.O.A.C. as owners and first flew on the 10th September, 1952. On the 23rd September, 1952 it was certified and approved by A.R.B. for the issue of its Certificate of Airworthiness and this Certificate, No. A.3221, was issued by the Ministry of Civil Aviation on the 30th September, 1952. After approval by A.R.B. on the 21st September, 1953 the Certificate of Airworthiness was renewed on the 23rd September, 1953 and was valid at the time of the accident".
Comet