No, you land at Heathrow you don't go pass passport control if you change onto another international flight. You go through security and then into the departure lounge.

Obviously if you land at Heathrow and fly out of Manchester then sure you have to immigrate, but not at the same airport.

Same at Dubai and Qatar, Helsinki and Frankfurt, Singapore. and Seoul.

Indeed last time I transited Seoul I stayed in a hotel airside for 12 hours before my next flight.