I have seen this all around, especially when I traveling in UK. As a ex-colony resident, I was particularly attracted to the Crown. Finally, I have done some digging:
經常在各地，尤其在英國，見到《保持冷靜，繼續前進》。作為一個前殖民地居民，皇冠徽號特別引起我注目，終於做了一些調查：
image source: wikipedia
"Keep Calm and Carry On was a motivational poster produced by the British government in 1939 in preparation for World War II. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities. "
"...the poster was hardly ever publicly displayed and was little known until a copy was rediscovered in 2000 at Barter Books, a bookshop in Alnwick. It has since been re-issued by a number of private companies, and has been used as the decorative theme for a range of products."
"《保持冷靜，繼續前進》是1939年第二次世界大戰開始時英國政府製作的宣傳海報，原計劃應對納粹佔領英國這一情況發生後，用以鼓舞民眾的士氣。由於這張海報張貼量有限，最初並不為人所知。直到2000年，在一家二手書店中被發現並張貼，才引起廣泛關注，至今相關設計被眾多商家印刷發行以作為產品的裝飾主題。"
So, I made something out of it from here: https://www.keepcalm-o-matic.co.uk
自己也搞一個玩玩：
Have fun & enjoy life!