Thank you both for your replies. Very helpful. I've been in both cities before, but since this summer's Olympics, London holds a little more interest plus I know it better, language and so.

I would fly ORD/LHR direct arriving 11:05 departing SAA one stop to CPT at 18:05. That's a seven hour layover. Would that seem like enough time to catch the tube to Central London and walk around a bit? With an hour each way allowing for immigration and so on, maybe that's tight. There are earlier ORD/LHR flights. At any rate, you've put me on the way to working this out. Thanks again.