As long as your layover is less than 24 hours you will not need to apply for a transit visa for the UK. As long as you have a visa for the US then that will act as a transit visa for the UK.
You will not however be allowed to leave the airport during your layover. If you do want to go out and about during your layover then you will need to apply for a transit visa or tourist visa.
As long as your layover is less than 24 hours you will not need to apply for a transit visa for the UK. As long as you have a visa for the US then that will act as a transit visa for the UK.
You will not however be allowed to leave the airport during your layover. If you do want to go out and about during your layover then you will need to apply for a transit visa or tourist visa.
You can find some more information on the Expat Arrivals page on Visas for the UK. To apply for visas for the UK visit the VFS Global South Africa website.
Hope that helps.