It is a very niche question and I doubt that many on these forums would be able to help you out. You may need to consult a solicitor that specialises in old (i.e. not current) nationality law. Please note that we are not allowed to recommend any solicitors or firms (or indeed providers of other services either) on these forums.
A quick Google search for "federation of rhodesia and nyasaland by double descent" or similar words give some further information, which you may have to look into in more detail.
This is a rare situation - resulting in a successful application as a British citizen - involving the Federation of Rhodesia and Nyasaland. This solution arises where:
- candidate born between 1 January 1964 and 31 December 1982;
- candidate's father or paternal grandfather born in Rhodesia;
- candidate's mother and maternal grandfather born outside of Rhodesia;
- candidate's parents married before 1 March 1958;
- a grandparent (paternal grandfather or maternal grandmother) born in the UK.
The candidate's mother becomes a Federal citizen, but does not become a citizen of Rhodesia upon dissolution. This CUKC status could be passed down by descent and, coupled with the Right of Abode, gives British citizenship to the candidate.
As you can see, the requirements are very specific. Keep in mind that this route to double descent was not planned as such, but is more an outcome of accident/happenstance in the formulation of a few different Acts of Parliament. It was a case of "Mind the Gap" between various individual pieces of legislation.
Further reference
The Federation of Rhodesia and Nyasaland (Dissolution) Order in Council 1963, in particular, see Section 74.
As you have at least one grandparent born in the UK, you are of course eligible for an
Ancestry visa, which would lead to British citizenship after five years of residence in the UK. That is not dependent on the location of the birth of your parents, so long as at least one grandparent was born in the UK (including the whole of Ireland before 1922).
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.