The examples that Le Tigre cite of reduced Africa services from LHR all have very different reasons, for example SAA had to cancel the CPT-London route, because leaving a brand new A330 unused for 12 hours between its overnight flights is hardly an effective use of the resource, and therefore covering the costs of buying/leasing it.
BA dropped DAR because they were not nimble and flexible enough in satisfying demand. Business traffic would be in and out of Dar Es Salaam, but tourist traffic is mainly for northern Tanzania and Zanzibar, and BA failed to even try and capture these markets, which are now being served by other carriers, like Turkish, ET etc. KLM has flown to Kilimanjaro for many years and does very well. And these routes attract premium leisure traffic as well.
BA could have started a triangle service with Zanzibar, JRO etc, but did not. Kigali would have been another city with which to partner with and so not using extra slots, and using the same aircraft.
On Harare, there were many reasons for pulling out, for example instability and the hatred of President Mugabe to most things British, but fuel was a major reason no doubt.
I do not accept that money cannot be made on African routes from LHR, after all much of Africa is booming, and KLM do very well indeed on them, and their colonial links there are far less than those of the UK!