from many countries and cultures sharing their written words.
The British have some really weird preposition uses. Except it's not at all weird, just not what I'm used to. I'm going to talk about style and specifics in a series of grammar posts. Because it truly is more like guidelines, anyway. And the only real rule is:
MAKE YOURSELF CLEARLY UNDERSTOOD.
FORWARD(S), BACKWARD(S), INWARD(S), OUTWARD(S)
Generally, The British like to add the "s" and the Americans and Canadians, don't.
That's it. There's no correct way. If you write for AP or The New York Times, they'll have a style manual for consistency and probably prefer no "s." I personally don't use an "s" but also don't give a crap if you do.
LESSON 1: If you are moving toward an anal insistence on perfect grammar, you are only really moving towards disappointment.