There must be a register of your birth somewhere and your adoptive parents would have to have registered you as an adopted child and for that you get a new birth certificate .
When an official Adoption Order is made, that entry on the Adopted Children�s Register supersedes any previous birth record entry. Therefore, for the purposes of obtaining official documents such as a passport or driving licence, an adopted person will need to obtain a copy of their entry in the Adopted Children�s Register. The Register is kept by the Registrar General, and contains a record of every person who has been adopted through a court in England or Wales since 1st January 1927.
If you have that bit of paper a solicitor cannot possibly tell you that don't exist .It's a legal document .I have one .
http://www.ukbirth-adoptionregister.com/resour ces.php