A departure from CPT at 07:30 would arrive at LHR at about 19:20 so would be in time to form another flight.
From aircraft rotation POV where to? Typical longhaul turn is around 2 hours and you can’t chuck a 380 onto any old route .... at the moment the last BA 380 leaves at just after 9 PM....(JNB do you want to delay that ...what are the implications for that service if the inbound CPT is delayed). The later departures at present the likes of ABV, BOM and the Middle East.
From a pax POV a late arrival into LHR might be ok for the Brits but it still leaves the thorny issue of connecting pax.