For example, why oh why do people use "of" instead of "have"?
It's NEVER "I should of…" or "would of". It's "I should HAVE" or "would HAVE".
I say that as someone who speaks several languages - and English is actually my second language. I don't think that people in Britain get taught English in school properly anymore. 
Does no one care anymore? I don't think anyone would get a job at my company making those mistakes, although I admit that writing is a big part of my job, so it really matters.
AIBU or do others get annoyed about this, too?