Hi,
I am not familiar with the UK adoption system, but perhaps my answer will help you even a little.
When adopting an infant from a private agency (I assume that is the scenario you are inquiring about) the potential adoptive parents will take the newborn child to their home after he or she has been released from the hospital, provided that the first parents have signed the adoption forms where they consent to the termination of their parental rights.
This is a legal document but it doesn't finalize the adoption. At least not in the adoption systems I am familiar with. In order for an adoption to be finalized, where the judge signs an adoption order after the court has established that the child has indeed been adopted legally and the adoptive parents are willing and able to raise the child. Usually there is a hearing that takes anything between 20 minutes to an hour.
When do there hearings and signing of papers occur? It varies from place to place and country to country. In some cases, the first parents will have a specific time frame when they can ask for the adoption consent to be revoked, but for the most part, once the consent has been signed (the document they sign a day or two after the baby is born so the adoptive parents may take him or her home) they can only have the baby back if they have been defrauded by the authorities or the adoptive parents. So, having to wait until the adoption process is finalized doesn't mean that the child can be taken back during this time.