First of all, I need to add the disclaimer that the following answer is not intended to be legal advice.
To adopt internationally through the US government (State Department and US Citizenship and Immigration Services [USCIS]), one member of the couple must be a US citizen. The problem you face is not so much with the adoption, but with the immigration of that child. To immigrate a child to the US, and thus to gain US citizenship, you must be a US citizen, which is logical when viewed from the immigration standpoint. Your child must be given citizenship, and the only citizenship you can confer is British. Therefore, you must work through the UK adoption and citizenship laws. Try contacting the Department for Children, Schools, and Families (DFSF) for information. Once your child has British citizenship you can apply to the USCIS to allow the child to live in the US.