Hi, Sam.
In the context of British operations, trains traveling toward London are UP while those headed away from London are DOWN.
Also, if trains are traveling on cross country or other lines whose location is not near London or traveling along them will not bring a train toward or away from London, the terms UP and DOWN will be determined based upon a designated principal station for that line or line(s).
Here's a link to an explanation which should provide additional clarification as the poster is a Driver (2nd answer posted from top):
http://in.answers.yahoo.com/qu...0091101225123AAmwsL1
I hope this helps.
Bob