What a Blimpish fuss about a perfectly reasonable and understandable question from an individual member of the armed forces - nothing to do with rank - at a public question and answer session arranged by the MoD and at which the media were present at MoD invitation.
If you don't want awkward questions, don't arrange such events.
Personally, I thought the Prime Minister's response was good, and I enjoyed the wry smile from the SoS.