Last week we had the following situation, a SUG of horse archers 2BW away from player's own table edge was charged from the front. The player wanted to run away by turning the unit 90 degrees to the left fleeing the charge zone towards his flank. The rules state that you may turn or wheel only to align in the direction of the charge and not perpendicular to it, my interpretation is that in all likelihood the SUG could only retreat directly to it's rear and being the player's rear table edge would have been removed from play, is this correct?

Basically, you were correct