There are a lot of complex laws relating to Southern Rhodesia and British nationality. You'd need a good immigration lawyer to work through everything, however on the surface it would appear she is NOT a British citizen because from 1949 Southern Rhodesia was treated as a "country" for nationality purposes.
Also, usually adoption outside the UK required registration as a UK citizen before age 18. Is there any evidence to suggest that this occurred?
HOWEVER, there may be the prospect of a Right of Abode stamp as long she has remained a "Commonwealth citizen" all the time since 1 January 1983. (Zimbabwe still counts as Commonwealth, as does South Africa since 1994).
http://www.ukvisas.gov.uk
If all else fails, then the Ancestry Visa may still be an option as I think adoptive relationships are still counted for that. There would need to be a UK born grandparent. But I would look at Right of Abode by preference.
Any chance the UK born grandparents come from Ireland or Northern Ireland?