You are probably right, I spoke from observation, not direct knowledge of the rules. I have seen a few "it's" online, and the British writers seemed to use it more consistently and appeared to be well-educated otherwise, while Americans used it less often and it seemed to be more a typo for them. Hence was an assumption born. This is the wrong forum for that!

It seems the British put the apostrophe in where Americans do not. It looks odd (=wrong) to us, but that's the way they are brought up. They're not going to change, so get used to it.

I grew up with "substantial" and noticed just a few years ago reading the newspaper that everyone suddenly started to use "substantive" for EVERYTHING. I don't think I've seen the word "substantial" in the paper since. Someone seems to have decreed its banishment from journalism. If we could find who that dictator is, perhaps we could reverse the process.
I was taught in school (long ago) that "substantive" meant a noun, and was a "grammar" word like "verb" or "subject", although I can see how it could also mean anything real or material. "Substantial" meant big or solid or important. As in, "we have substantial evidence to convict...", which would not be the same as "we have substantive evidence to convict," although the average person would not note any nuance between them.