Only British Airways goes direct to and from Cape Town all year round, providing a night flight service. Virgin fly directly only in season, commencing late Oct/early Nov to late Apr which is going to cease in Apr 15, service is also night flights. Night flights depart from London Heathrow in the evening and land in Cape Town the following morning.
Direct outbound to Cape Town flight time is 12 hours and returning a little shorter at 11 hours. During the UK summer time South Africa is only 1 hour ahead of us but when the clocks change in the UK that increases to 2 hours.
Airlines such as KLM, Air France, Lufthansa etc… offer very competitive rates requiring a flight change at their hub/base airport. These airlines also offer flights to Cape Town out of other UK airports such as Manchester via their hub/base airport therefore avoiding Heathrow.
South African Airways fly to/from Cape Town via Johannesburg. Booking early enough ahead there are some good deals where the domestic leg Johannesburg to/from Cape Town is offered free of charge.