411A
I'm sure someone at LHR has got wind of the fact that crews are by-passing the system: they are claiming their bag is going in the hold on the Manifest whilst actually putting it in the cabin (with an oversize bottle of shampoo etc).
The only way to police this is to have Security check the contents of cabin bags at the aeroplane. I would imagine even the DfT can't justify the extra manpower required for the job. SO, the onus has been placed on the Captains with the implicit threat of loss of (ID) livelihood if someone actually succeeds in by-passing the system.
How are the DfT going to police this unless a Security person physically examines the contents of each cabin bag before boarding? It is not the Captain's responsibility to examine the contents of cabin-crew baggage.