There's good availability to NBO if Kenya fits the bill. It has varied safari options and the coast too.
There's also a few dates available coming back from JNB (nothing outbound). I see you can fly from NBO to Livingstone (Vic Falls) so you could book LON-NBO//JNB-LON with two regional flights, crossing Zambia-Zim overland at the falls. But check out the flight timetable and costs first - could be prohibitive.