Somewhat late but, not sure how or why you decided Marxism is not something Oxford has an "affinity" towards. Aside from the fact there is an entire dedicated paper in the PPE Final Honour School on Marx & Marxism, and Marx is included in a separate paper on the history of political thought too.
Marxist approaches are pretty bog-standard social theory toolkit approaches used basically across the social sciences and humanities and I think you'd find it hard to complete a degree in those areas at a university of Oxford's calibre without at least having a basic idea of where and why those approaches get used (even if you choose not to utilise those approaches yourself, or indeed choose to critique them). Likewise research into colonialism, postcolonial approaches, and concepts of neocolonialism I think would also be pretty common to find. Not sure about pan-Africanism specifically but I imagine if you're studying African regions and/or politics you might be looking at a variety of approaches and movements which may include that.
Just searching through the staff flagged under "political theory" in the politics department and I found three people specifically referring to Marx and/or colonialism in their research interests. Just before COVID the African Society held a conference on Pan-Africanism hosted by the African Studies department there. I think you're making a lot of assumptions about what is taught and researched at Oxford and I'm not sure those reflect the reality of contemporary academia in general.
That said, based on the topics you note of interest, you may (also) want to explore anthropology as a potential degree programme rather than PPE, since I suspect a number of those and related topics would be featured there. Oxford offers it through archaeology and anthropology, while Cambridge offers it through HSPS (which allows you to also do some sociology and/or politics). The topics you've expressed interest in seem less strongly tied to either philosophy or economics to me.