If the up direction from Carlisle station is to London or Newcastle, and the up direction at Newcastle is to London or Carlisle, which is up and which down at Hexham? Answers on a postcard to somebody else, please.

If you think of Up as implying superior, I think that explains the motivation behind all the usages. Up North and Down South, that feels right. As for Oxford, that's an internal Southern matter, so only Southerners care.