And often not even then in my experience - T5 Heathrow is where (maybe) all BA's internal flights leave from and a huge amount of their international. Same security, same lounges, same terminal - only time ID is needed is before getting on the plane internationally (or at the Visa-Check desk if that applies to your destination). Mind you a I read something about them taking a photo of you at security for internal flights, and 'assuring' you it's deleted once the flight has left.