There are organisations which specialise in dealing with cases where someone has been adopted or fostered, including:
BAAF: (British Agency for Adoption and Fostering)
Now under the aegis of the Coram charity, BAAF can help with issues and tracing concerning adoption and fostering. https://corambaaf.org.uk
International Social Services:
International Social Service is for relatives of people missing overseas can help with adoptees and others seeking contact with birth parents and vice versa.
The Adoption Contact Register,
The register is available at the General Register Office, contains a record of every person who has been adopted through a court in England or Wales. They might be able to help with information on applying for adoption certificates, receiving information on original birth details and advice on making contact with adopted people and their relatives. They can also register an interest in wanting to be found (by both parties).
TALK adoption is an organisation supporting young people up to the age of 25 who are adoptees, their siblings, friends or birth parents. Advice is given on tracing relatives, rights, birth identity etc.
Whilst this information has been provided in good faith, it should not be taken as legal advice. For information tailored to your circumstances, please contact your police force, solicitor or an advisory organisation as appropriate to your query.