Originally Posted by Pilot37

From my experiences (C-W-S can confirm), if traveling HBO to the US and starting in LHR, T5 security scan won't let you pass through the initial BP scanner (if on your phone) until this has been done (essentially it is part of the 'Fit to Fly' process), so you need to do it land-side before passing through security. If you are transferring via T5 from another flight and you didn't have contact with an agent at your original point of origin, it is done at Flight Connections before you go up the escalator to security in T5A.

Pilot37