Hypothetical situation: What nationality would this child be?

Say a child was born in Britain to a British mother and American father, and had duel citizenship in both countries. The child's parents then divorced and the mother married a British man, who adopted the child. Therefore the American biological father has relinquished parental responsibility and legal rights over the child, so in a legal sense, he is of no relation to the child. Would the child still have both American and British citizenship? Or would it now be a citizen of Britain only?