Upvote:1
"Common format" refers to the physical form of your permit, not the category. If your permit is a plastic card with a bull on it, it is a common-format permit.
Furthermore, even if your residence permit is not in the common format, your valid US visa exempts you from the DATV requirement regardless of your destination.
See https://www.gov.uk/check-uk-visa/y/south-africa/transit/somewhere_else/no:
You donβt need a visa if you have one of the following:
- a visa for Canada, New Zealand, Australia or the USA (this can be used for travel to any country)
...
- a common format residence permit issued by an European Economic Area (EEA) country or Switzerland