Why change up and down?
Because it is not clear and requires more route knowledge to understand.
However.......I don’t know how to consistently improve it....
Up and down works with London to an extent, but do northerners think “going down to London”?
London-bound is clearer, but longer, and what would the opposite be - “country bound” or “Exeter bound”, “Bristol bound” etc.
Some lines have clear north/south/east/west bound directions......but some less direct lines could get a bit scruffy on that.
Always Up to London, or the principle destination. Spent 53 years working on the railway and never had any problem with understanding that.
Yes, there are some abnormalities, the lines from Great Yarmouth, Cromer and Lowestoft to Norwich are Up until they get to Thorpe Junction at Norwich when they become Down for the final bit in to the platforms because the lines from London form the Down. It's no great problem to Railwaymen, sorry, Railpersons.