There's no arrival duty free at CPT but anything you might want to buy at LHR is much cheaper in any shop in Cape Town, especially given the fall in the Rand. South Africa is also one of the few places where customs officers will fine you for being slightly over the personal allowance. It's also worth remembering that you can't buy duty free if you are travelling on to another country in the Southern Africa Customs Area (e.g. Botswana, Namibia etc). Don't forget that you can reclaim VAT when you leave the country which will be worth it if you go mad in the shops.