You probably need to write-off/export the bike from the Kenya then. But ask from Kenyan autoclub (if they have one) or from South African autoclub that is it possible to export the Kenyan plated bike out from SA. So you can ride it to SA without carnet and do the export procedure there (well, IF it's possible).
The problem is, if you try to import the bike into your destination (home) country, it has to be written off from the origin country's register the bike was from, it can't be registered in two countries, then it's registered into your country and plated.
Also very probably you need to pay your countrie's tax on importing from non-EU country. Check it how much it is. But there are also few tricks how to make the "market worth" price lower (i.e. take few critical bits off in the SA and say it's a broken bike back in the UK etc), so the tax you have to pay will be lower.