Not sure of what people know but adoption was not official until about 1913 so any before then I am not sure what the process would have been.
I have just found that one of mine may have been adopted in some form. She has on her death certificate her natural mother's name and she has a birth certificate logged by her mother but clearly written in the area of marriage and father "illigitimate". Only last night I decided to look for her first marriage and found she was using another surname, further search an aunt was quoted in the paper as saying I know this man he married my neice, only to find this lady's maiden name was the names quoted on the marriage certificate.
through the address on the marriage certificate and that of the Electoral Roll I found the family she was with but on her "adopted" father's death she is not mentioned. Her adopted parents had lost a baby just before her birth and another the year after her birth so I am fairly sure she was with them.
Just looking for examples of what may have occured as she knew who her biological mother was on later documents.