Boris's image control people have clearly advised to bring on the Brigadier in order to re-draw the Churchill/winning of world wars kind of feelings (those that led to the Brexit vote when we were all enjoying the First World War centenary celebrations from 2014). They know that the British public are always unwilling to criticise the armed forces. Well played to Boris's PR team.
Boris himself, however, could easily have been replaced by Rowley Birkin QC from the Fast Show and none of us would have noticed.