4 February 1920
By the 1920s, the development of heavier than air flight had progressed considerably, partly due to the outbreak of World War One. Britain, having emerged victorious from this war also possessed a large store of aircraft and personnel trained in their use and maintenance.  At the time  the British Empire was the largest in the world, and thus governed countries quite a distance from England itself. Advances made in aircraft during WW1, potentially made administering these far flung colonies much more convenient. It was in this spirit that the British Air Force began exploring routes to the Far and Middle East, Australia and the British Colonies in Africa. Flight was however still considered the domain of states and the military. Thus the private attempt launched on 4 February 1920 to open the London to Cape Town route was  met with great sceptism by the British authorities. However once the endeavour seemed underway, the British authorities hastily joined the expedition. Unfortunately this expedition was not to be successful as the technologies required were not adequately developed and all four aircraft that took off from London subsequently crashed on the African continent.  Two South African pilots, Lt-Col Pierre van Ryneveld and Capt. CJQ Brand, were part of the expedition that made it to Cape Town. This was accomplished by replacing one of the downed aircraft, and after a further crash, borrowing an aircraft from the South African Air Force.
References

Churchill. W et al, (1920), Imperial Air Routes: Discussion, The Geographical Journal, Vol. 55, No. 4 (Apr., 1920), pp. 263-270 Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of British Geographers)|McCormack. R. L, (1989) Missed Opportunities: Winston Churchill, the Air Ministry, and Africa, 1919-1921The International History Review, Vol. 11, No. 2 (May, 1989), pp. 205-228