Adult Concept.

I read in DH that magic school isn't compulsory. So it is OK if I create an adult British character that hasn't graduated from a magic school, but is a pretty good witch?

I'm not an admin so I can't answer this question, but you should post this in the Question & Answer board. An admin will look at it faster there then here in the sandbox.

I have moved this to the proper board.

I am not sure where you read that in DH because during that year magic school WAS compulsory for non-muggleborns.  Either way, schooling of some sort would be needed.  If your history can back up some sort of schooling, even if it is homeschooling or tutors, then it can be considered.  However if you were planning on making a recent graduate (IE: a half or pure blood who would have been a 7th year during DH) then they would have been required to go to Hogwarts during their 7th year.

Thanks