Am looking for something myself. It has been a long established ambition to approach by sea to take in the view. There used to be a line Union Castle (or Castle something) I think it was called - I saw it featured on some in-house DVDs onboard QM2. Would love to hear any information.
I think you're going to have real problems trying to get a cruise from the UK to Cape Town in Dec 07. There isn't one that doesn't involve flying from what I can see. Good luck in your search.