Just to add a few shades of reality to this debate
Rachel Reid: With a nudge and a wink, MoD has dragged me through the mud | Comment is free | The Guardian
So two meetings at Nato HQ in which the agenda was civilian casualties. The only possible reason for these figures to be classified is as CirrusF says because they were supplied by an ally, for which read: the US.
This is typical over-classification of data - which leads the whole system into disrepute - and this stinks of a British officer being arrested, yet again, on the say-so of his US counterparts as per Stankovic.
She confirms that she received information from the officer. She does not state the security classification of the material. She does however say that her lawyers have intimated that it is not covered by the OSA.
Absolute non story then. Obviously the information was not classified at all (otherwise it very definitely WOULD be covered). The lady doth protest too much, methinks.