If you start in MRU and fly to Europe then, on your return to Africa via the Atlantic, you cannot fly LHR-MRU, because the second entry in Europe rule only permits flights to certain countries in Africa:
Originally Posted by oneworld.com
(e) Only one intercontinental departure and one intercontinental arrival permitted in each continent except
as follows:
1. ...snip
2. ...snip
3. For travel to/from or via Ghana, Nigeria, Kenya, Uganda and Tanzania two permitted in Europe when one is a transfer without stopover between Ghana/Nigeria/Kenya/Uganda/Tanzania and another continent.
Edited to add: if you show your segments like this MRU-LHR instead of MRU/LHR they are a little easier to read.