Your adoption certificate, if you were adopted ater 1928, will have been issued at the local register office where your original birth entry was made, (most likely) and so if you contact them direct they will explain the procedure (quite simple) and you should be able to order a copy of your Origianal certificate for �7.
this site explains the regulations and the records http://www.familyrecords.gov.uk/topics/adoptio n.htm