Generally Wages Staff were those who got dirty and Salaried Staff those who didn't, althouth there were exceptions.
At a workshop for instance Wages Staff were the fitters, machine operstors, floor sweepers, etc. Salaried Staff were Shop Foremen, Supervisors, Clerks, Work Study men, etc.
I can remember places where the canteen was segregated, for instance Goddard Street canteen at Crewe Works had a big hall and a queue for those in overalls and small room where two ladies brought the food to the table for those in suits.