I had always assumed that was an example of WWII British propaganda that has its uses in that war along with aircraft recognition posters, "loose lips sink ships" and "is this journey really necessary?'. Not forgetting thsee misogynistic anti-VD posters,"she may look clean". I didn't know that these posters were locked away only to used in event of "aquatic feline" aka Unternehmen Seelöwe. In the nature of advertising there were earlier versions of this poster, "Keep Calm and Carry On", that didn't fully meet with client approval as seen below. It makes one wonder about what Strangelovian materials were created "in the event of" for the edification of those who remained.
The images / screen captures of the earlier versions of KCaCO are from the documentary Ian Hislop's Stiff Upper Lip - An Emotional History Of Britain.