Originally Posted by Globaliser

The photo confirms that you are "inside" the UK for immigration purposes. This is the case if you have arrived at T5 on a domestic flight. It is also the case if you have checked-in landside at T5 and then entered the airside area through security.

The confirmation is needed because once you are beyond security (if coming from landside), or once you have entered the departure lounge (if you are connecting from a domestic flight), you will be mixing with passengers who have arrived at T5 on international flights, who have not cleared immigration, and are therefore "outside" the UK for immigration purposes. The photo system allows both "inside" and "outside" passengers to mix before boarding, and means that only those who are "inside" the UK are allowed to board an onward domestic flight. This is why the photo that was taken earlier is checked when you board the onward domestic flight.

But this is why you must have your photo taken within the transit route before you leave the domestic arrivals stream and enter the departure lounge, even though there is no need for you to clear security again.