Originally Posted by Genius1

After check-in, the first desks (the temporary ones for now) are staffed by BA agents who will scan your boarding pass to determine TFW/lounge eligibility.

Behind the desk are the automated boarding pass readers to gain entry to the airside part of the terminal, alongside a desk with a HAL member of staff who will deal with guests, or passengers unable to use the gates (as they do today at North and South Security).

BA have a presence before the transition to airside effectively. BA are not (and cannot be under DfT regulations) responsible for security or controlling access to airside, hence the HAL presence.