There should be a fair bit of info on the site about shipping from SA to UK, it's a pretty common thing so search a few of the shipping posts and see what turns up. Options include air/sea from Cape Town, air from Joberg, roro from Durban (specifically I think Miquel Silvestre has used that service and posted comments again recently). I shipped out of SA but not to the UK, however can provide details on crating services if you require.
In regards to shipping into Kenya, haven't heard much. Once you factor in costs and hassle of shipping in and out for a 6 week trip, you can see why people start to consider rentals or buying there. The bikes aren't cheap though, but it's probably worth while considering as well. Ride safe.