<p>There are several possible origins: </p>

• In train parlance, "Up" trains go towards London, "Down" trains go away from London.
• Going "Uptown" vs. going "Downtown" which refers to specific sections of London, rather than London itself. Belgrave square and other places the Crawleys would tend to hang out would be considered "uptown".

<p>It is a rather alien concept to Americans since the US is not dominated by a single large city, the way England and many other European countries are. Thus, in America, up and down tend to simply align with directions on a compass. There are exceptions of course, as going to the mountains is generally up, and the shore is generally down. Philadelphian's are known for the phrase "Down the shore", although the cardinal direction is basically East. </p><p>At least that is the understanding of a Yank. :) </p>

<p>Here in northern England, the term "down to London" is used very frequently,  probably more so than  "up to London". </p><p>But the upper classes and wealthy business people are more inclined to say "up to London". </p>