Get rugged up over your swimmers :)
You'll probably need to be rugged up when you leave London. It's be pretty fresh and wet until you get doen into Spain. A few layers should sort you out though.
I can't speak for the rest of your African adventure (very envious!!), but South Africa in August is fabulous. Not balmy, but comfortably warm (albeit a bit windy). It may get a bit chills overnight, but then that will be the case for Northern Africa too.
You know what, unless we're talking flash flooding, and monsoons, (experienced African adventurers will be able to let you know), the most important thing is that you are ready. No point rushing it and forcing things.
If you don't have bugs in your teeth, you haven't been grinning enough!