I would 100% support RustyRefs last paragraph. I also worked in the City and my worst performances were certainly midweek. I was rarely "late" to a game, but if I was struggling for time it would always be midweek. Relying on Southern Trains to get me home from work so that I could then venture out (often using the M25) for a game was a high-risk activity.
I don't know if that embeds an advantage for referees from the midlands and the north, but it was hard work and without an understanding boss would have caused me to quit sooner than I did.
I do also think that there is a skewing of top non-league sides towards the south, meaning the referees in this part of the world had more challenging games and fewer opportunities to "shine". We always believed, however, that "the system" worked and that the cream would rise to the top.