You’ve seen it in your posters, Facebook, magazines and news paper’s and yes it gives us that British feeling of pride. Along with the personality of the British,.  No matter how bad it gets, you keep calm and carry on. 

So, the original poster was made during WW2.  Maybe it just fitted the criteria of the times to keep the public faith. Anyway, the poster never actually got distributed. It was a chance find out of the few copies that were left, and now seen as a decorative piece of art, brought in a few years back.