I am flying Virgin Atlantic from the USA back home to South Africa. I have a 13-hour layover at Heathrow airport. Will I be able to pass through passport control to be able to go smoke in the designated areas, and then return through security before my flight back to South Africa?
From the new rules that activated yesterday, these parts were REMOVED from the rules...
you’re travelling from (or on part of a reasonable journey from) Australia, Canada, New Zealand or the USA and have a valid visa for that country
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
Visitor in Transit Strategy: You only need a valid visa for entry into the USA (Australia, etc). Being on the leg of a trip to/from the USA (Australia, etc) is now irrelevant. The US visa, or any other exemption document listed in the rules, such as a valid common format Category D visa for entry to an EEA state or Switzerland is sufficient to qualify.
Temporary Admission Strategy: Failing all of that, it is also perfectly acceptable to ask an Immigration Officer for a Temporary Admission. There's paperwork, and they will take custody of your passport, but it's an avenue if all else fails. They do not have to give a TA if they don't want to, and they do not have to have a good reason for refusing. However it does NOT count as a refusal that must be reported in subsequent applications.