Cape Town has had a reputation for high crime rates in the past but the authorities have taken substantial steps to reduce crime and improve safety, especially with regard to tourist safety. Nearly half a million Britons visit South Africa each year and most visits are trouble free. Common sense will ensure your safety in Cape Town, its not advised to walk around alone after dark, try not to carry large amounts of money or have cameras/camcorders dangling from backpacks etc. When travelling in Cape Towns surounding areas, extra care might be needed. The standard of driving in South Africa can vary so be aware if you plan on driving. Stay in company and avoid remote beaches and picnic areas. Most of the crime occurs in the townships, so if you are planning to travel into these locals will be able to advise of the places to avoid. Table Mountain National Park offers some wonderful sites but be vigilant in the quieter areas of the park early in the morning or just before the park closes. Hijacking, whilst uncommon, poses a threat to visitors so don't pick up anyone, motorists in 'distress' is a common ploy in hijackings - rather than stopping to help, its much better to inform the Police (10111 or 112 from Cell phones).
A point to note if you plan on travelling to Table Mountain, the Table Mountain National Park suffered bush fires in late January. Lifts to the summit are now operating again and the park is open but some paths to and from the summit will be unsafe until they are cleared - wardens will be able to say if this has been done.
Remember that many Britons visit Cape Town every year and tourists are very well looked after and it is uncommon to be the victim of crime. Have a great trip!