For the US military, it is the same.
If it says anything other than "Unclassified" on the top and bottom, it's the start of a very bad day for the individual divulging the material, intentionally or otherwise. Similarly, it is NOT up to the individual to decide if something is or is not classifed according to his whims.
"Due process?" By all means, but even then, the US military rules (UCMJ) are different than in the civilian code.
In the case of this British Lt Col, I would think that only if the officers are in either his direct chain of command or on his court martial proceedings would be 'predjudicial (sp?)' to the defendant.
Again, if interesting, your argument about the 'embarrassment' factor or 'keeping in with the Americans' is moot. The officer either did or did not divulge classified information to one not authorized to have that information.
The FOIA, whether the information should be classified at all, etc, etc, is fine for discussion, but at the time the officer divulged the information, it was. That seems to be the merits of the incident.