its been like that since the days of VC10's at both JNB and CPT
the overnight flight from LON stays there until the evening to head back home
back in the 2000's BA started a double daily 747 to/from CPT one of which was a day flight home leaving around 0830 from CPT
if there are 3 a/c there this morning then one maybe tech from yesterday>? or is one a LGW flight now with a 777
as for traffic BA def seem to have enough (more than enough) especially since both SAA and VAA gave up the route but VAA was summer only anyway
now BA has some competition with Thomas cook and condor plus EK and TK via their mid-east hubs - plus KQ too via NBO
LH and KLM have done the CPT route for many years all year round and AF is summer