In BE, the noun is usually written either make-up or make up. The verb, to make up, can mean either to put make up on or, as in Parla's example above, to invent. (It has other meanings too - see the WR dictionary).Makeup meaning things like lipstick is one word, not two.
Make up, two words, is a phrasal verb: "She made up an excuse for being late to work."
If you google "make up uk" you'll find plenty of examples where it is written as two words - for example:In BE the noun is usually written "make-up" (or "makeup"). I don't think you'll find it written as two words. British dictionaries support this view.
I don't feel the need to look, being one who considers that a dictionary's role is to describe use rather than prescribe it; if educated native speakers such as Donny B and me use it, that's enough for me!. Do you have any dictionary evidence, Susan?
I was simply (post #2) answering the OP's question as asked.I did notice Donny's "make up", and it made me wince a little. Donny is of the male persuasion, I think. Do you have any dictionary evidence, Susan?
I don't mean that we never see it written as two words, but it's unusual enough to stand out - as it did for me with Donny's post.