The UK’s immigration law has a provision which allows British Citizens to automatically pass on their citizenship to any adopted children. However, the adoption must be in line with the Hague convention for this provision to be valid. It appears that in your case the adoption was not in line with the Hague convention, this does not invalidate your adoption, but it does explain the Home Office refusing your application. I will include the government guidance which explains this matter, please note it is not exhaustive: https://www.gov.uk/government/publications/guidance-on-how-adopted-children-can-become-british/intercountry-adoption-and-british-citizenship.