Is that on one ticket? If you're on two separate tickets, what's your backup plan?

One possible backup plan is to have 26:25 between the flights rather than 2:25 - particularly if having two separate tickets is likely to mean no through-checking by SA onto BA, with a consequent need to clear immigration, collect bags, clear customs and then get to the BA check-in desk in time for the T-60m bag drop cut-off.

Alternatively, if you're flying on a day when there's a morning or lunchtime flight from WDH to CPT, take that instead of the one that you're currently planning.

My daughter and I also did the VIP Fast Track at JNB, connecting to MUB. It was well worth the extra cost, as we were immediately taken to the front of the immigration line and sailed on through. Yes, I did feel guilty passing the line, but was surely glad to be through and on our way. It helped our recovery time once we reached our final destination.

I'd love to know more about this. Cost, company name and so on if you wouldn't mind?

Are queues usually long then?

I hope you enjoy F! One of my first BA trips was in F on a 747 (back in the early 90's) and i recall it as spectacular, well that is the memory of a very excited flyer at the time. Flying in the nose felt very special.
Separately I have had the same issue at JNB and did the overnight at the Intercontinental airport hotel there which I thought was great and a very short walk to the terminal.

Notice you've made your mind up, good. Just to chime in as posted by others the UD on the 380 isn't great. I actually don't like it. F in the nose of a 747 is always great :) Enjoy.

The company we used was Bidair, we booked and paid online. It was around £50 for me and Mr RR at the time but it will depend on the exchange rate at the time of booking. The queues can be very very long and usually when we have been through ORT, not enough staff on duty can add to a lengthy wait. We have been through ORT 6 times in the last 5 years and will be going again next year when connecting to WDH and will use Bidair again. We found them very easy to deal with.
We were met by a gentleman just after we got off the aircraft, guided to the front of the said queues, then onto baggage reclaim. From departing the aircraft to the car hire reception desk was approx 20 minutes.
BidAir

To the extent that "bottled" might have a pejorative connotation, I don't think it's warranted. That sounds like a very sensible compromise: it doesn't delay your return by as much as 24 hours (even though it sounds like it wouldn't cause you any difficulty), and if you're taking an extra day to complete your holiday, then a day flight has many benefits in any event.