Have the ORR actually got a definition of what “CDL” is exactly? Has it got to involve a physical lock? Is is explicitly forbidden that secondary locks cannot be manually operated by stewards under the ‘central’ instruction of the guard?
For example, 2nd generation multiple unit sliding doors are not ‘locked’ per se, unless they are manually locked out of use with a carriage key. They are just held shut with air pressure. It’s quite possible to pull them open in motion if you’re strong enough, although this would trigger an emergency brake application. It’s also possible to open them, easily, at any moment using the emergency door release handle which any passenger has access to, placed above the door. Any passenger could, if they wish, operate this handle and open a door in traffic, which would be easier to do than lowering a droplight and opening the handle on the outside. People generally don’t, as there is a sign warning them of big fines if used inappropriately, as there could be for slam door stock.