Red is a color traditionally associated with England and later Great Britain. For example, English military uniforms of the early modern period were often red, and St. George's Cross (which features prominently on both the old English flag and the modern British Union Flag) is also red. That said, it's hard to read black-colored text that's printed on a red background, so pink is a natural choice for a printer who still wants Britain and her possessions to be inked in an appropriate color.

Warm colors come to the foreground, while cool colors recede in the visual hierarchy. So the object of the mapmaker's attention—what he wished people to notice first—would usually be colored red if small linework or type, and pink (a screen or hatching of red) if it's a large area where ink coverage (on the presses of the era) or legibility of overprinting type would be a problem. A German or Chinese or Brazilian cartographer would almost certainly make the same choice. In addition, of course, the oceans on such maps were often printed in blue, leaving fewer options for political distinctions.

As a professional cartographer, I use these design principles every day. It sometimes leads to disagreements with my clients who hire me to do bicycling maps. Thinking it's mnemonic, they often think the routes should be coded red-means-stop; yellow-means-caution; green-means-good-for-cycling. But this produces maps in which the most prominent things you see are the streets you shouldn't bicycle on, which sort of defeats the purpose in producing a map to promote cycling.