Upvote:5
If you are on a single ticket and going international-international then you do not go through immigration. Check http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/terminal-2-international-connecting-to-terminal-2-international for a same terminal example or http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/terminal-5-international-connecting-to-terminal-3-international for a multiple terminal example.