WHBM
The 744s must be fully depreciated so there is virtually no capital cost. This is not true of the 380s two of which spend all day in JNB - but as mentioned elsewhere a little extra yield will make up for higher capital costs. A departure from CPT at 07:30 would arrive at LHR at about 19:20 so would be in time to form another flight. I believe that BA did this many years ago, presumably the stopped this for a reason.
I'm not sure that BA aircraft are utilised particularly heavily. Have a look at Swiss International - there does not seem to be much slack in their schedules at all. Or Air Canada (and I suspect many other airlines) during the summer holidays. Looking at FR24 Edelweiss has four long haul aircraft which fly for six days in the week with less than two hours between most sectors, although they do rest on the seventh day. One advantage of long layovers is being able to catch up time - as long as you have rested crews waiting.
I have to say that when flying back from HKG or the far east I always try and take the day flight. There is nothing worse than arriving at 05:30 after a 13 hour flight. Arriving at 10:00 after a 7 hour eastbound flight is bearable. I should say that I regretably generally fly economy these days.