Upvote:2
You are correct: you are exempt. You can find the wording on the UK Government web site at https://www.gov.uk/check-uk-visa/y/south-africa/transit/somewhere_else/yes.
Transiting without a visa
You might be eligible for ‘transit without visa’ if:
- you arrive and depart by air
- have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive
- have the right documents for your destination (eg a visa for that country)
One of the following must also apply:
...
- you’re travelling from (or on part of a reasonable journey from) Australia, Canada, New Zealand or the USA and it’s less than 6 months since you last entered that country with a valid entry visa