Leek Town used to be in the Southern League, long before the SL added its Central division, despite being north east of Stoke on Trent. I once saw them play Sittingbourne in that competition.
But Bradford surely have the record for the most northern team ever to play in the SL. Bradford played in the Southern League in 1907/08 and were elected to FL D2 at the end of the season despite only finishing 13th. Undeserving though this was, Manningham RL, aka Bradford City, were elected in 1903 without ever having kicked a round ball. Chelsea are the only other club to be elected to the FL without having played a game of soccer.