What are the actual legal procedures for adoption in the UK? I need reliable sources and links (preferably from a government organization or a .org)

I'm trying to see how likely Harry Potter's adoption to the Dursleys is.

I need stuff such as, if there is home study, if they go over family history, if a will is needed, etc.

Can someone really leave a relative on a doorstep without a will to prove it and legal procedures?