Why do you assume that the fares will rise between now and next year?

Like you, I like to be organised, but this has bitten me on the backside more than once with booking almost a year out and IME J class fares behave very differently to the cheap Y fares.

This is what I see looking for 4 seats JNB-LON.

For the week after next.
£833 on MK (this allows a free stop in MRU...)
£976 on EY
£1022 QR
£1139 EK

Go to late July next year
£833 on MK
£975 EY
£1139 EK
£1224 QR

That MK fare looks very interesting if I have your itinerary right - it could well save you a boat load of hassle and potentially some £s. Get a refund on your JNB-MRU-JNB flights (160k Avios and £264) and you save 180k Avios and £900 on not coming back BA.

So you're 340k Avios up and £1164 in pocket. You then pay out £3332 to MK. Net result is you get 340k Avios for £2168. 0.63p per Avios which is exactly the same rate you can spend them on hotels. Or to put it another way, holding out for the BA redemption means you're valuing your Avios at 0.63p...

(if your kids are under 12 then the MK fare falls by £150 for each of them)