The basics
Here's the unusual thing about this route: despite being an 11-hour flight, the time difference is only 1 to 2 hours (depending on whether British Summer Time is in effect). You fly almost due south, crossing latitude rather than longitude. Your circadian clock barely needs to adjust.
So why do you feel terrible when you arrive?
Because what you're experiencing isn't really jet lag. It's travel fatigue. Eleven hours in a pressurised cabin at 35,000 feet, breathing 15% humidity air, sleeping in an upright seat, and arriving dehydrated and stiff takes a real physical toll regardless of time zones.
Most people recover fully within 24 to 48 hours. This isn't about clock adjustment. It's about physical recovery.
Travel fatigue vs jet lag
True jet lag gives you insomnia at night and sleepiness at wrong times of day. Travel fatigue gives you generalised exhaustion, headaches, muscle aches, and brain fog, but your sleep schedule stays roughly on track. You'll feel tired at a normal bedtime and able to sleep through the night. You'll just feel rough the next day.
The distinction matters because the recovery strategy is different. With real jet lag, light timing and melatonin are the tools. With travel fatigue, it's hydration, movement, and rest.
Recovery plan
On arrival, hydrate aggressively. Aim for 2 to 3 litres of water on your first day. Get outside for a walk, even if it's just 30 minutes. Sunlight and movement are the best antidotes to that flat, foggy feeling. Eat a proper meal at local time. Go to bed at your normal time. No need for melatonin since the time shift is negligible.
On day one, you might still feel slightly off. A mild headache, some fatigue. This is residual dehydration and cabin pressure effects. Another day of good hydration, light exercise, and normal eating usually sorts it.
By day two, most people feel completely normal.
What helps
Hydration is the main lever. The cabin air is incredibly dry and you lose more water than you realise during an 11-hour flight. Start hydrating before you board and keep drinking water throughout.
Movement after landing. Your body has been cramped for 11 hours. A walk, some stretching, or a swim (South Africa has no shortage of pools) gets your circulation going and speeds recovery.
Normal sleep schedule. Since the time difference is negligible, just go to bed and wake up at your normal times. Your body clock doesn't need resetting.
What to avoid
Don't mistake travel fatigue for jet lag and start taking melatonin or messing with your light exposure. Your clock is fine. Your body needs recovery.
Don't sleep the entire day when you arrive, thinking you have "jet lag." A short nap is fine; a 6-hour sleep will leave you groggy and disrupt your night.
Choosing your flight time
Most UK–South Africa flights depart in the evening (7 to 9pm) and land the next morning, which works reasonably well against normal sleep timing since you're not fighting a big time shift. Try to actually sleep for a meaningful stretch of the flight rather than staying up watching films. How rough you feel on arrival is almost entirely about how much real sleep you got in the air, not about your body clock.
Malaria medication can complicate things
If your trip includes a malaria area (parts of the Kruger region, for example, though Cape Town itself is malaria-free), some antimalarial medications can affect sleep quality or cause vivid dreams as a side effect. If you're taking one for the first time, start it a few days before departure as advised anyway. That way any sleep-related side effects show up before you're relying on good sleep to recover from the flight.
Early safari starts make travel fatigue worse
If your trip includes a safari, game drives typically start before sunrise, often at 5am or earlier. Stacking a 5am wake-up on top of unresolved travel fatigue from the flight is a common reason people feel worse on day two or three than they did on arrival. Where you can, build in one full rest day before an early-start safari segment rather than going straight from the airport to the bush.
Coming home
Same thing in reverse. Minimal time change, significant travel fatigue. Hydrate, rest, and you'll be fine within a day.