Question Author
B.C.C. answered my Email with, "the City Council does not have any surviving adoption records from the 1940s", so no, they didn't give me a reason as to how/why it was lost.
I have my father's original Birth Certificate, which includes his mother's full unmarried name, the address of where he was born, her occupation and the address she was residing in at that time. Unfortunately, his father's name is not mentioned. I also have the adoption letters from a Miss Primmer (from B.C.C.) to my grandparents regarding him having check-ups etc. I also have a handwritten letter from Dad's birth mother to my grandparents asking how he was and for photos. Her address was c/o someone in South Devon in 1942. Any help in this matter will be gratefully received.