In your opening post, you mention you have a 'copy' of a birth certificate.
When was that certificate issued?
If it was issued at the time of the birth registration, it would not record a later adoption.
If the certificate was issued years after the birth by at least 21 years and is still not annotated on the right hand side that an adoption has been recorded, then it could be safe to assume he wasn't adopted.
To be sure, you could order a copy now from the General Register Office and see what the certificate contains.