Thanks.
I already have seats on 58/59 in CW which rules out JNB (Mrs Pomkiwi would not be pleased with a stop-over just to get an upgrade

) and will be very happy with that. I have lots of Avios and would be happy using a few to travel F (especially outbound).
I was mainly interested in that this seems an easy proposition at the moment on one pair of flights but impossible on the other.