I am a Vietnamese traveller, without a landside transit visa, arriving at Heathrow with ongoing direct travel to the USA via Gatwick with a valid USA Visa. My ticket to the USA leaves within the transit visa exempt time limit, but I will be meeting a friend outside the Heathrow terminal and there could be a possibility that I over-extend the exempt visa time limitation.
I'm assuming I could take a later flight to the USA but what are the consequences? How do they stamp my passport in Heathrow? Can they prevent me from taking a later flight? What can they do to me? etc.