Originally Posted by
ajcopley
I've flown through Heathrow many times connecting on to the EU, and in this situation you do not go through UK immigration simply because you've landed in the UK.
That's true. But the UK permits international-to-international connections without being admitted to the U.K. Everyone arriving in the U.S. must be admitted at the airport of entry (which might be a pre-clearance facility) before being allowed to access a connecting flight.