It's not to differentiate between Highlanders and Englishmen. It's to differentiate between Highlanders and Turks!

Pretty much all northern troops have bonuses in the snow, while southern soldiers get bonuses in the desert.

Infantry get bonuses in the forest, if they come from forested areas, which gives them an extra advantage over cavalry, which suck in the woods, and those southern desert dwellers.

When taken in a limited context like the Britannia campaign, it might not seem immediately obvious, but it makes perfect sense.