United Kingdom - Passport & Nationality - British Citizenship (Adopted outside the UK) Over 18

Adoption outside of the UK
If a child was adopted outside the UK, then one needs to look at the natural (or biological) parents to determine whether the adopted child has a claim to British Nationality. The nationality of the adopting parents will normally have no bearing on the nationality of the child, though again there are some exceptions.

2.1 Adoptions before 1983

Where an adoption took place before 1 January 1983 in a country recognised by the UK for this purpose, the adopted child can obtain the Right of Abode if one of the adoptive parents was a British citizen. 

2.2 Adopted children who are still under 18

For as long as an adopted child is still a minor (under the age of 18), then this adopted child could be registered as a British citizen if one of the adoptive parents is a British citizen. It is a complex application, and is granted by discretion.

2.3 An adoptive grandparent born in the UK

If a parent of one of the adoptive parents was born in the UK, the adopted child could still qualify for the UK Ancestry if they are a) 17 or over and b) hold a Commonwealth passport.