The agreed Heathrow/Border Force process is that any customers leaving the IDL should be escorted via international arrivals. During mass disruption, there can and do allow for domestic passengers to go via a domestic gate but this has to be agreed and IIRC, there is a requirement for agents to verify that you are registered on a domestic service. It should also be a Heathrow agent who escorts passengers out of the IDL.

If you are taken via a domestic channel, that would be authorised by Heathrow/Border Force. Then the Heathrow agent should present you to the security desk at the domestic connection point for your biometrics to be checked and delogged.