I have believed, on the basis of a book read many years ago, that airways were bidirectional, with flights in opposing direction separated vertically by 1000 feet. For example, northbound flights are assigned an even number of thousands of feet, southbound an odd number (or vice versa). East and west get the odd 500s.
The reason most most north-south and south-north flights follow the M6 rather than the A1 is to avoid Lincolnshire and its relatively unpredictable military aviation.
There may some amongst us with ATC experience who can confirm this (or explain why I am spouting nonsense).