Sorry but you are not understanding the point here. That vestibule is not for passenger use all because it has easy access to emergency equipment and the cab. Nowadays any crew area is strictly out of bounds to passengers. It's not really a case of ow the train is crowded so we will let the passengers into the crew areas. The cab is only lockable from the vestibule when the gangway connection is in use as the door that seals the gangway swings over to seal the cab when the gangway connection is in use. So it is not possible to lock out the cab from the vestibule unless using the gangway connection. The sliding lockable door the OP went through is the door that prevents access to the cab. Also the OP has admitted they knew it was wrong to go into that area.
I don't think you're understanding properly:
On classes 317 and also 150/2, there is the passenger saloon, then a sliding door, then two single leaf side doors (in a short corridor that when this unit is leading is out of bounds). There is then another door, that can either be closed to be parallel with other internal doors, or moved through 90 degrees to close off the driving cab only.
When this cab is in the middle, this door is shut to close the driving position only, and the gangway door is opened to close-off the secondmans' position. The internal buttons (same as at the other passenger doors) will open these. On the FCC 317s the exteral buttons were plated over several years ago.
On 313 and 315, there is the passenger saloon, then a sliding door, then an L shaped area that includes a single leaf door on both sides and also inclides the secondmans' front window. There is then a drivers cab which is locked off again.
These people were in the L shaped area. When 313 and 315 were new, the rear/middle areas were routinely available for passenger standing.
On all the above, the exterior door can be opened from the outside with a bit of force.
Perhaps our class 455 friend can confirm their arrangements. I understand that the lack of some door or another on SWT is the reason why passengers cannot walk between 455 units.