I'm planning to go from the US to Cape Town some time in 2019. The plan will be a night flight to Heathrow, stay in a hotel for the day (depending on how awake we are), then night flight to Cape Town. On the return, I will stay in London for a few weeks to break it.
A piece of advice here: to book something like this (with breaks), one typically has to use the "multi-city" option on a travel or airline website. If you do it on an airline's website, you usually get crazy prices - they appear to treat it like several one-way tickets. The same for some intermediary websites (booking.com/expedia.com), etc. If I search via a multi-intermediary site like Kayak, I get way better prices (less than half sometimes).