Yes it is allowed, there are two big signs in the check-out area (pictures in the linked threads somewhere), and in the non-fast track queues there is are often staff members with a pillar like gizmo on wheels, and a trolley, to give you a luggage tag on the spot. But you best go to the marked desk (at LHR) just by North or South security. This is instead of a checked allowance and not "as well as", you need to be going through security with just the small personal item. LGW should do in in normal check-in or at the machines on the side.