Flightstats shows a US POS. BA will be showing availability for the origin location instead.
The risk is no availability, not being able to change your flight and your current backup one way fare not being available or more expensive.
As far as I can see the one-way WTP fare for JNB-LHR books into a W fare-bucket and is fully flex with no cancellation penalties ... So you could book that then call up after your outward to see if you get lucky with availability into a T or E fare-bucket to re-price the inward leg of your original ticket (I am assuming here this was originally booked into a T fare-bucket) and then cancel the one-way ticket.
The other problem you are going to face though trying to get T fare-bucket availability is a possible 7 day advance purchase requirement (in place at least for the current T-fare for the LHR-JNB-LHR route). I am not sure, but I am guessing this also applies to flight changes.