Travel to /from SA didn't really happen until the autumn because they shut their borders except for repatriation. Since then know a few people who have been back to see family, so reckon it probably appeared around Oct/Nov hence being picked up in Dec. Loads of people fly via Dubai, its usually cheaper.
I can remember saying to DP in the summer that as soon as SA reopen the borders to Europe the cases will surge, which they did. Didn't anticipate this coming back the other way.
I really hope this helps, it would be easy to say this is a pointless exercise as it must be everywhere by now but as far as I can see this is not just about making sure people isolate but also collecting data which presumably can then be extrapolated to understand more about the spread.
I'm not in one of the postcodes, but very close by, and I hope as many people as possible comply. Our rates have been falling fast from a total disaster zone before Xmas.