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.
LHR (like many other foreign airports) separates arriving and departing passengers and provides transit paths. In almost all USA airports, there's no way to separate arriving and departing passengers, with all passengers except international arrivals (terminating at that airport) having access to all gates.