Dont know about July but you can do a world cruise sector in January. P&O Aurora leaves for Capetown on 11 Jan, 17 nights incl overnight flight back but you can extend your stay in SA.
Fred. Olsen's "Black Watch" sailing on its world cruise from Southampton on January 06, 2009 calls at Durban and Capetown in mid March. You can book the sector from Sydney (Australia) to Capetown as a fly-cruise. The departure date from the UK is 20 February. See the Fred. Olsen 2009/2010 brochure or visit their website for fuller details.